<PAGE>

                                                                    Exhibit 4.6



                TRUST PREFERRED SECURITIES GUARANTEE AGREEMENT


                              AGL Resources Inc.



                             Dated as of [_______]
<PAGE>

                               TABLE OF CONTENTS
                                  (continued)

<TABLE>
<CAPTION>
                                                                                 Page
<S>                                                                              <C>
                                    ARTICLE I
                         DEFINITIONS AND INTERPRETATION

SECTION 1.1    Definitions and Interpretation.................................      1

                                   ARTICLE II
                               TRUST INDENTURE ACT

SECTION 2.1    Trust Indenture Act; Application...............................      5

SECTION 2.2    Lists of Holders of Securities.................................      5

SECTION 2.3    Reports by the Guarantee Trustee...............................      5

SECTION 2.4    Periodic Reports to Guarantee Trustee..........................      5

SECTION 2.5    Evidence of Compliance with Conditions Precedent...............      6

SECTION 2.6    Events of Default; Waiver......................................      6

SECTION 2.7    Event of Default; Notice.......................................      6

SECTION 2.8    Conflicting Interests..........................................      7

                                   ARTICLE III
                 POWERS, DUTIES AND RIGHTS OF GUARANTEE TRUSTEE

SECTION 3.1    Powers and Duties of the Guarantee Trustee.....................      7

SECTION 3.2    Certain Rights of Guarantee Trustee............................      8

SECTION 3.3    Not Responsible for Recitals or Issuance of Guarantee..........     10

                                   ARTICLE IV
                                GUARANTEE TRUSTEE

SECTION 4.1    Guarantee Trustee; Eligibility.................................     11

SECTION 4.2    Appointment, Removal and Resignation of Guarantee Trustee......     11

                                    ARTICLE V
                                    GUARANTEE

SECTION 5.1    Guarantee......................................................     12

SECTION 5.2    Waiver of Notice and Demand....................................     12

SECTION 5.3    Obligations Not Affected.......................................     12

SECTION 5.4    Rights of Holders..............................................     13

SECTION 5.5    Guarantee of Payment...........................................     14

SECTION 5.6    Subrogation....................................................     14

SECTION 5.7    Independent Obligations........................................     14
</TABLE>

                                      -i-
<PAGE>

                               TABLE OF CONTENTS
                                  (continued)

<TABLE>
<CAPTION>
                                                                                 Page
<S>                                                                              <C>
                                   ARTICLE VI
                    LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 6.1    Limitation of Transactions.....................................     14

SECTION 6.2    Ranking........................................................     15

                                   ARTICLE VII
                                   TERMINATION

SECTION 7.1    Termination....................................................     15

                                  ARTICLE VIII
                 COMPENSATION AND EXPENSES OF GUARANTEE TRUSTEE

                                   ARTICLE IX
                                 INDEMNIFICATION

SECTION 9.1    Exculpation....................................................     16

SECTION 9.2    Indemnification................................................     16

                                    ARTICLE X
                                  MISCELLANEOUS

SECTION 10.1   Successors and Assigns.........................................     17

SECTION 10.2   Amendments.....................................................     17

SECTION 10.3   Notices........................................................     17

SECTION 10.4   Benefit........................................................     18

SECTION 10.5   Governing Law..................................................     18
</TABLE>

                                     -ii-
<PAGE>

                                                                     Exhibit 4.6

                TRUST PREFERRED SECURITIES GUARANTEE AGREEMENT


This Trust Preferred Securities Guarantee Agreement (the "Guarantee"), dated as
of [__________], is executed and delivered by AGL Resources Inc., a Georgia
corporation (the "Guarantor"), and The Bank of New York, a New York banking
corporation, as trustee (the "Guarantee Trustee"), for the benefit of the
Holders (as defined herein) from time to time of the Trust Preferred Securities
(as defined herein) of AGL Capital Trust III, a Delaware statutory business
trust (the "Trust").

WHEREAS, pursuant to an Amended and Restated Trust Agreement (the "Trust
Agreement"), dated as of [___________], among the trustees of the Trust, AGL
Capital Corporation, as sponsor, and the holders from time to time of undivided
beneficial interests in the assets of the Trust, the Trust is issuing on the
date hereof preferred securities, having an aggregate liquidation amount of
$[_________], such Trust Preferred Securities being designated the [ ]% Trust
Preferred Securities (the "Trust Preferred Securities").

WHEREAS, as incentive for the Holders to purchase the Trust Preferred
Securities, the Guarantor desires irrevocably and unconditionally to agree, to
the extent set forth in this Guarantee, to pay to the Holders the Guarantee
Payments (as defined below). The Guarantor agrees to make certain other payments
on the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the purchase by each Holder, which purchase
the Guarantor hereby acknowledges shall benefit the Guarantor, the Guarantor
executes and delivers this Guarantee for the benefit of the Holders.

                                   ARTICLE I
                        DEFINITIONS AND INTERPRETATION

          SECTION 1.1  Definitions and Interpretation

In this Guarantee, unless the context otherwise requires:

               (a)    Capitalized terms used in this Guarantee but not defined
                      in the preamble above have the respective meanings
                      assigned to them in this Section 1.1;

               (b)    Terms defined in the Trust Agreement as of the date of
                      execution of this Guarantee have the same meaning when
                      used in this Guarantee;

               (c)    a term defined anywhere in this Guarantee has the same
                      meaning throughout;

               (d)    all references to "the Guarantee" or "this Guarantee" are
                      to this Trust Preferred Securities Guarantee as modified,
                      supplemented or amended from time to time;

                                       1
<PAGE>

          (e)   all references in this Guarantee to Articles and Sections are to
                Articles and Sections of this Guarantee, unless otherwise
                specified;

          (f)   a term defined in the Trust Indenture Act has the same meaning
                when used in this Guarantee, unless otherwise defined in this
                Guarantee or unless the context otherwise requires; and

          (g)   a reference to the singular includes the plural and vice versa.

"Affiliate" has the same meaning as given to that term in Rule 405 under the
 ---------
Securities Act of 1933, as amended, or any successor rule thereunder.

"Business Day" means any day other than a Saturday or a Sunday, or a day on
 ------------
which banking institutions in The City of New York are authorized or required by
law or executive order to close.

"Common Securities" means the securities representing common undivided
 -----------------
beneficial interests in the assets of the Trust.

"Corporate Trust Office" means the office of the Guarantee Trustee at which the
 ----------------------
corporate trust business of the Guarantee Trustee shall, at any particular time,
be principally administered, which office at the date of execution of this
Guarantee is located at 101 Barclay Street, 21 West, New York, New York 10286.

"Covered Person" means any Holder or beneficial owner of Trust Preferred
 --------------
Securities.

"Debentures" means the series of subordinated debt securities of AGL Capital
 ----------
Corporation designated the [ ]% Junior Subordinated Deferrable Interest
Debentures due [______] held by the Property Trustee of the Trust.

"Event of Default" means a default by the Guarantor on any of its payment or
 ----------------
other obligations under this Guarantee.

"Guarantee Payments" means the following payments or distributions, without
 ------------------
duplication, with respect to the Trust Preferred Securities, to the extent not
paid or made by the Trust: (i) any accumulated and unpaid Distributions (as
defined in the Trust Agreement) that are required to be paid on such Trust
Preferred Securities to the extent the Trust has funds on hand legally available
therefor at such time, (ii) the redemption price, including all accumulated and
unpaid Distributions to the date of redemption (the "Redemption Price") to the
extent the Trust has funds on hand legally available therefor at such time, with
respect to any Trust Preferred Securities called for redemption by the Trust,
and (iii) upon a voluntary or involuntary termination and liquidation of the
Trust (other than in connection with the distribution of Debentures to the
Holders in exchange for Trust Preferred Securities as provided in the Trust
Agreement), the lesser of (a) the aggregate of the liquidation amount and all
accumulated and unpaid Distributions on the Trust Preferred Securities to the
date of payment, to the extent the

                                       2
<PAGE>

Trust has funds on hand legally available therefor, and (b) the amount of assets
of the Trust remaining available for distribution to Holders in liquidation of
the Trust.

"Guarantee Trustee" means The Bank of New York, a New York banking corporation,
 -----------------
until a Successor Guarantee Trustee has been appointed and has accepted such
appointment pursuant to the terms of this Guarantee and thereafter means each
such Successor Guarantee Trustee.

"Holder" shall mean any holder, as registered on the books and records of the
 ------
Trust, of any Trust Preferred Securities; provided, however, that, in
determining whether the holders of the requisite percentage of Trust Preferred
Securities have given any request, notice, consent or waiver hereunder, "Holder"
shall not include the Guarantor or any Affiliate of the Guarantor.

"Indemnified Person" means the Guarantee Trustee, any Affiliate of the Guarantee
 ------------------
Trustee, or any officers, directors, shareholders, members, partners, employees,
representatives, nominees, custodians or agents of the Guarantee Trustee.

"Indenture" means the Indenture dated as of [________], among AGL Resources
 ---------
Inc., AGL Capital Corporation (the "Debenture Issuer") and The Bank of New York,
as trustee (the "Property Trustee"), pursuant to which the Debentures are to be
issued to the Property Trustee of the Trust.

"Indenture Event of Default" shall mean any event specified in Section 5.01 of
 --------------------------
the Indenture.

"Majority in liquidation amount of the Trust Preferred Securities" means, except
 ----------------------------------------------------------------
as provided by the Trust Indenture Act, a vote by Holder(s) of more than 50% of
the aggregate liquidation amount (including the stated amount that would be paid
on redemption, liquidation or otherwise, plus accumulated and unpaid
Distributions to the date upon which the voting percentages are determined) of
all Trust Preferred Securities.

"Officer's Certificate" means, with respect to any person, a certificate signed
 ---------------------
by the Chairman, the Chief Executive Officer, the President, a Vice President,
the Controller, the Secretary or an Assistant Secretary, the Treasurer or an
Assistant Treasurer of the Guarantor. Any Officers' Certificate delivered with
respect to compliance with a condition or covenant provided for in this
Guarantee shall include:

               (a)   a statement that each officer signing the Officers'
         Certificate has read the covenant or condition and the definitions
         relating thereto;

               (b)   a statement that each such officer has made such
         examination or investigation as, in such officer's opinion, is
         necessary to enable such officer to express an informed opinion as to
         whether or not such covenant or condition has been complied with; and

               (c)   a statement as to whether, in the opinion of each such
         officer, such condition or covenant has been complied with.

                                       3
<PAGE>

"Other Debentures" means all junior subordinated debentures issued by the
 ----------------
Guarantor or the Company from time to time and sold to trusts to be established
by the Guarantor or the Company (if any), in each case similar to the Trust.

"Other Guarantees" means all guarantees to be issued by the Guarantor with
 ----------------
respect to Trust Preferred Securities (if any) similar to the Trust Preferred
Securities issued by other trusts to be established by the Guarantor or the
Company (if any), in each case similar to the Trust.

"Person" means a legal person, including any individual, corporation, estate,
 ------
partnership, joint venture, association, joint stock company, limited liability
company, trust, unincorporated association, or government or any agency or
political subdivision thereof, or any other entity of whatever nature.

"Redemption Price" has the meaning provided therefor in the definition of
 ----------------
Guarantee Payments.

"Responsible Officer" shall mean, when used with respect to the Guarantee
 ------------------
Trustee, any officer within the corporate trust office of the Guarantee Trustee,
including any vice-president, any assistant vice-president, assistant secretary,
assistant treasurer, trust officer or any other officer or agent of the
Guarantee Trustee who customarily performs functions similar to those performed
by the Person who at the time shall be such officer, respectively, or to whom
any corporate trust matter is referred because of such person's knowledge of and
familiarity with the particular subject and who shall have direct responsibility
for the administration of the Trust.

"Successor Guarantee Trustee" means a successor Guarantee Trustee possessing the
 ---------------------------
qualifications to act as Guarantee Trustee under Section 4.1.

"Trust Indenture Act" means the Trust Indenture Act of 1939, as amended.
 -------------------

"Trust Securities" means the Common Securities and the Trust Preferred
 ----------------
 Securities, collectively.


                                  ARTICLE II
                              TRUST INDENTURE ACT

          SECTION 2.1  Trust Indenture Act; Application

                (a)    This Guarantee is subject to the provisions of the Trust
Indenture Act that are required to be part of this Guarantee and shall, to the
extent applicable, be governed by such provisions; and

                (b)    if and to the extent that any provision of this
Guarantee limits, qualifies or conflicts with the duties imposed by Section 310
to 317, inclusive, of the Trust Indenture Act, such imposed duties shall
control.

          SECTION 2.2  Lists of Holders of Securities

                                       4
<PAGE>

               (a)     The Guarantor shall provide the Guarantee Trustee (unless
the Guarantee Trustee is otherwise the registrar of the Trust Preferred
Securities) with a list, in such form as the Guarantee Trustee may reasonably
require, of the names and addresses of the Holders ("List of Holders") as of
such date, (i) not more than 15 days after each record date for payment of
Distributions, and (ii) at any other time within 30 days of receipt by the
Guarantor of a written request for a List of Holders as of a date not more than
15 days before such List of Holders is given to the Guarantee Trustee provided,
                                                                      --------
that the Guarantor shall not be obligated to provide such List of Holders at any
time the List of Holders does not differ from the most recent List of Holders
given to the Guarantee Trustee by the Guarantor. The Guarantee Trustee may
destroy any List of Holders previously given to it on receipt of a new List of
Holders.

               (b)     The Guarantee Trustee shall comply with its obligations
under Sections 311(a), 311(b) and Section 312(b) of the Trust Indenture Act.

          SECTION 2.3  Reports by the Guarantee Trustee

Within 60 days after May 15 of each year, commencing May 15, [___], the
Guarantee Trustee shall provide to the Holders such reports as are required by
Section 313(a) of the Trust Indenture Act, if any, in the form and in the manner
provided by Section 313 of the Trust Indenture Act. The Guarantee Trustee shall
also comply with the other requirements of Section 313 of the Trust Indenture
Act.

          SECTION 2.4  Periodic Reports to Guarantee Trustee

The Guarantor shall provide to the Guarantee Trustee such documents, reports and
information as required by Section 314 (if any) and the compliance certificate
required by Section 314 of the Trust Indenture Act in the form, in the manner
and at the times required by Section 314 of the Trust Indenture Act provided
that such compliance certificate shall be delivered on or before 120 days after
the end of each fiscal year of the Guarantor. Delivery of such reports,
information and documents to the Guarantee Trustee is for informational purposes
only and the Guarantee Trustee's receipt of such shall not constitute
constructive notice of any information contained therein or determinable from
information contained therein, including the Guarantor's compliance with any of
its covenants hereunder (as to which the Guarantee Trustee is entitled to rely
exclusively on Officers' Certificates).

Delivery of such reports, information and documents to the Trustee is for
informational purposes only and the Trustee's receipt of such shall not
constitute constructive notice of any information contained therein or
determinable from information contained therein, including the Trust's
compliance with any of its covenants hereunder (as to which the Trustee is
entitled to rely exclusively on Officer's Certificates).

          SECTION 2.5  Evidence of Compliance with Conditions Precedent

The Guarantor shall provide to the Guarantee Trustee such evidence of compliance
with any conditions precedent, if any, provided for in this Guarantee that
relate to any of the matters set forth in Section 314(c) of the Trust Indenture
Act. Any certificate or opinion required to be

                                       5
<PAGE>

given by an officer pursuant to Section 314(c)(1) may be given in the form of an
Officers' Certificate.

          SECTION 2.6  Events of Default; Waiver

The Holders of a Majority in liquidation amount of Trust Preferred Securities
may, by vote, on behalf of all the Holders, waive any past Event of Default and
its consequences. Upon such waiver, any such Event of Default shall cease to
exist, and any Event of Default arising therefrom shall be deemed to have been
cured, for every purpose of this Guarantee, but no such waiver shall extend to
any subsequent or other default or Event of Default or impair any right
consequent thereon.

          SECTION 2.7  Event of Default; Notice

               (a)     The Guarantee Trustee shall, within 90 days after the
occurrence of a default with respect to this Guarantee, mail by first class
postage prepaid, to all Holders, notices of all defaults actually known to a
Responsible Officer of the Guarantee Trustee, unless such defaults have been
cured before the giving of such notice, provided, that, except in the case of
default in the payment of any Guarantee Payment, the Guarantee Trustee shall be
protected in withholding such notice if and so long as the board of directors,
the executive committee, or a trust committee of directors and/or Responsible
Officers of the Guarantee Trustee in good faith determines that the withholding
of such notice is in the interests of the holders of the Trust Preferred
Securities.

               (b)     The Guarantee Trustee shall not be deemed to have
knowledge of any Event of Default unless the Guarantee Trustee shall have
received written notice from the Guarantor or a Holder, or a Responsible Officer
of the Guarantee Trustee charged with the administration of the Trust Agreement
shall have obtained actual knowledge, of such Event of Default.

          SECTION 2.8  Conflicting Interests

The Trust Agreement shall be deemed to be specifically described in this
Guarantee for the purposes of clause (i) of the first proviso contained in
Section 310(b) of the Trust Indenture Act.

                                  ARTICLE III
                POWERS, DUTIES AND RIGHTS OF GUARANTEE TRUSTEE

          SECTION 3.1  Powers and Duties of the Guarantee Trustee

               (a)     This Guarantee shall be held by the Guarantee Trustee for
the benefit of the Holders, and the Guarantee Trustee shall not transfer this
Guarantee to any Person except a Holder exercising his or her rights pursuant to
Section 5.4(b) or to a Successor Guarantee Trustee on acceptance by such
Successor Guarantee Trustee of its appointment to act as Successor Guarantee
Trustee. The right, title and interest of the Guarantee Trustee shall
automatically vest in any Successor Guarantee Trustee, and such vesting and
succession of title shall be effective

                                       6
<PAGE>

whether or not conveyancing documents have been executed and delivered pursuant
to the appointment of such Successor Guarantee Trustee.

          (b)  If an Event of Default actually known to a Responsible Officer of
the Guarantee Trustee has occurred and is continuing, the Guarantee Trustee
shall enforce this Guarantee for the benefit of the Holders.

          (c)  The Guarantee Trustee, before the occurrence of any Event of
Default and after the curing of all Events of Default that may have occurred,
shall undertake to perform only such duties as are specifically set forth in
this Guarantee, and no implied covenants shall be read into this Guarantee
against the Guarantee Trustee. In case an Event of Default has occurred (that
has not been cured or waived pursuant to Section 2.6) and is actually known to a
Responsible Officer of the Guarantee Trustee, the Guarantee Trustee shall
exercise such of the rights and powers vested in it by this Guarantee, and use
the same degree of care and skill in its exercise thereof, as a prudent person
would exercise or use under the circumstances in the conduct of his or her own
affairs.

          (d)  No provision of this Guarantee shall be construed to relieve the
Guarantee Trustee from liability for its own negligent action, its own negligent
failure to act, or its own willful misconduct, except that:

               (i)  prior to the occurrence of any Event of Default and after
the curing or waiving of all such Events of Default that may have occurred:

                       (A)  the duties and obligations of the Guarantee Trustee
shall be determined solely by the express provisions of this Guarantee, and the
Guarantee Trustee shall not be liable except for the performance of such duties
and obligations as are specifically set forth in this Guarantee, and no implied
covenants or obligations shall be read into this Guarantee against the Guarantee
Trustee; and

                       (B)  in the absence of bad faith on the part of the
Guarantee Trustee, the Guarantee Trustee may conclusively rely, as to the truth
of the statements and the correctness of the opinions expressed therein, upon
any certificates or opinions furnished to the Guarantee Trustee and conforming
to the requirements of this Guarantee; but in the case of any such certificates
or opinions that by any provision hereof are specifically required to be
furnished to the Guarantee Trustee, the Guarantee Trustee shall be under a duty
to examine the same to determine whether or not they conform to the requirements
of this Guarantee. The Trustee shall not be bound to make any investigation into
the facts or matters stated in any resolution, certificate, statement,
instrument, opinion, report, notice, request, direction, consent, order, bond,
debenture, note, other evidence of indebtedness or other paper or document, but
the Trustee, in its discretion, may make such further inquiry or investigation
into such facts or matters as it may see fit, and, if the Trustee shall
determine to make such further inquiry or investigation, it shall be entitled to
examine the books, records and premises of the Guarantor, personally or by agent
or attorney at the sole cost of the Guarantor and shall incur no liability or
additional liability of any kind by reason of such inquiry or investigation;

                                       7
<PAGE>

               (ii)   the Guarantee Trustee shall not be liable for any error of
judgment made in good faith by a Responsible Officer of the Guarantee Trustee,
unless it shall be proved that the Guarantee Trustee was negligent in
ascertaining the pertinent facts upon which such judgment was made;

               (iii)  the Guarantee Trustee shall not be liable with respect to
any action taken or omitted to be taken by it in good faith in accordance with
the direction of the Holders of a Majority in liquidation amount of the Trust
Preferred Securities relating to the time, method and place of conducting any
proceeding for any remedy available to the Guarantee Trustee, or exercising any
trust or power conferred upon the Guarantee Trustee under this Guarantee; and

               (iv)   no provision of this Guarantee shall require the Guarantee
Trustee to expend or risk its own funds or otherwise incur personal financial
liability in the performance of any of its duties or in the exercise of any of
its rights or powers, if the Guarantee Trustee shall have reasonable grounds for
believing that the repayment of such funds or liability is not reasonably
assured to it under the terms of this Guarantee or indemnity, reasonably
satisfactory to the Guarantee Trustee, against such risk or liability is not
reasonably assured to it.

     SECTION 3.2  Certain Rights of Guarantee Trustee

          (a)  Subject to the provisions of Section 3.1:

               (i)    The Guarantee Trustee may conclusively rely, and shall be
fully protected in acting or refraining from acting, upon any resolution,
certificate, statement, instrument, opinion, report, notice, request, direction,
consent, order, bond, debenture, note, other evidence of indebtedness or other
paper or document believed by it to be genuine and to have been signed, sent or
presented by the proper party or parties.

               (ii)   Any direction or act of the Guarantor contemplated by this
Guarantee may be sufficiently evidenced by an Officers' Certificate.

               (iii)  Whenever, in the administration of this Guarantee, the
Guarantee Trustee shall deem it desirable that a matter be proved or established
before taking, suffering or omitting any action hereunder, the Guarantee Trustee
(unless other evidence is herein specifically prescribed) may, in the absence of
bad faith on its part, request and conclusively rely upon an Officers'
Certificate which, upon receipt of such request, shall be promptly delivered by
the Guarantor.

               (iv)   The Guarantee Trustee shall have no duty to see to any
recording, filing or registration of any instrument (or any rerecording,
refiling or registration thereof).

               (v)    The Guarantee Trustee may consult with counsel of its
selection, and the advice or opinion of such counsel with respect to legal
matters shall be full and complete authorization and protection in respect of
any action taken, suffered or omitted by it hereunder in good faith and in
accordance with such advice or opinion. Such counsel may be counsel to the
Guarantor or any of its Affiliates and may include any of its employees. The
Guarantee Trustee

                                       8
<PAGE>

shall have the right at any time to seek instructions concerning the
administration of this Guarantee from any court of competent jurisdiction.

               (vi)   The Guarantee Trustee shall be under no obligation to
exercise any of the rights or powers vested in it by this Guarantee at the
request or direction of any Holder, unless such Holder shall have provided to
the Guarantee Trustee such security and indemnity, reasonably satisfactory to
the Guarantee Trustee, against the costs, expenses (including attorneys' fees
and expenses and the expenses of the Guarantee Trustee's agents, nominees or
custodians) and liabilities that might be incurred by it in complying with such
request or direction, including such reasonable advances as may be requested by
the Guarantee Trustee; provided that, nothing contained in this Section
3.2(a)(vi) shall be taken to relieve the Guarantee Trustee, upon the occurrence
of an Event of Default, of its obligation to exercise the rights and powers
vested in it by this Guarantee.

               (vii)  The Guarantee Trustee shall not be bound to make any
investigation into the facts or matters stated in any resolution, certificate,
statement, instrument, opinion, report, notice, request, direction, consent,
order, bond, debenture, note, other evidence of indebtedness or other paper or
document, but the Guarantee Trustee, in its discretion, may make such further
inquiry or investigation into such facts or matters as it may see fit.

               (viii) The Guarantee Trustee may execute any of the trusts or
powers hereunder or perform any duties hereunder either directly or by or
through agents, nominees, custodians or attorneys, and the Guarantee Trustee
shall not be responsible for any misconduct or negligence on the part of any
agent or attorney appointed with due care by it hereunder.

               (ix)   Any action taken by the Guarantee Trustee or its agents
hereunder shall bind the Holders, and the signature of the Guarantee Trustee or
its agents alone shall be sufficient and effective to perform any such action.
No third party shall be required to inquire as to the authority of the Guarantee
Trustee to so act or as to its compliance with any of the terms and provisions
of this Guarantee, both of which shall be conclusively evidenced by the
Guarantee Trustee's or its agent's taking such action.

               (x)    Whenever in the administration of this Guarantee the
Guarantee Trustee shall deem it desirable to receive instructions with respect
to enforcing any remedy or right or taking any other action hereunder, the
Guarantee Trustee (i) may request instructions from the Holders of a Majority in
liquidation amount of the Trust Preferred Securities, (ii) may refrain from
enforcing such remedy or right or taking such other action until such
instructions are received, and (iii) shall be protected in conclusively relying
on or acting in accordance with such instructions.

               (xi)   The Guarantee Trustee shall not be liable for any action
taken, suffered, or omitted to be taken by it in good faith, without negligence,
and reasonably believed by it to be authorized or within the discretion or
rights or powers conferred upon it by this Guarantee.

                                       9
<PAGE>

          (b)  No provision of this Guarantee shall be deemed to impose any duty
or obligation on the Guarantee Trustee to perform any act or acts or exercise
any right, power, duty or obligation conferred or imposed on it in any
jurisdiction in which it shall be illegal, or in which the Guarantee Trustee
shall be unqualified or incompetent in accordance with applicable law, to
perform any such act or acts or to exercise any such right, power, duty or
obligation. No permissive power or authority available to the Guarantee Trustee
shall be construed to be a duty.

          (c)  Whether or not therein expressly so provided, every provision of
this Guarantee relating to the conduct or affecting the liability of or
affording protection to the Guarantee Trustee shall be subject to the provisions
of this Section.

     SECTION 3.3  Not Responsible for Recitals or Issuance of Guarantee

The recitals contained in this Guarantee shall be taken as the statements of the
Guarantor, and the Guarantee Trustee does not assume any responsibility for
their correctness. The Guarantee Trustee makes no representation as to the
validity or sufficiency of this Guarantee.

                                  ARTICLE IV
                               GUARANTEE TRUSTEE

     SECTION 4.1  Guarantee Trustee; Eligibility

          (a)  There shall at all times be a Guarantee Trustee which shall:

               (i)  not be an Affiliate of the Guarantor; and

               (ii) be a corporation organized and doing business under the laws
of the United States of America or any State or Territory thereof or of the
District of Columbia, or a corporation or Person permitted by the Securities and
Exchange Commission to act as an institutional trustee under the Trust Indenture
Act, authorized under such laws to exercise corporate trust powers, having a
combined capital and surplus of at least 50 million U.S. dollars ($50,000,000),
and subject to supervision or examination by Federal, State, Territorial or
District of Columbia authority. If such corporation publishes reports of
condition at least annually, pursuant to law or to the requirements of the
supervising or examining authority referred to above, then, for the purposes of
this Section 4.1(a)(ii), the combined capital and surplus of such corporation
shall be deemed to be its combined capital and surplus as set forth in its most
recent report of condition so published.

          (b)  If at any time the Guarantee Trustee shall cease to be eligible
to so act under Section 4.1(a), the Guarantee Trustee shall immediately resign
in the manner and with the effect set out in Section 4.2(c).

          (c)  If the Guarantee Trustee has or shall acquire any "conflicting
interest" within the meaning of Section 310(b) of the Trust Indenture Act, the
Guarantee Trustee and Guarantor shall in all respects comply with the provisions
of Section 310(b) of the Trust Indenture Act, subject to the penultimate
paragraph thereof.

                                       10
<PAGE>

     SECTION 4.2  Appointment, Removal and Resignation of Guarantee Trustee

          (a)  Subject to Section 4.2(b), the Guarantee Trustee may be appointed
or removed without cause at any time by the Guarantor except during an Event of
Default.

          (b)  The Guarantee Trustee shall not be removed in accordance with
Section 4.2(a) until a Successor Guarantee Trustee has been appointed and has
accepted such appointment by written instrument executed by such Successor
Guarantee Trustee and delivered to the Guarantor.

          (c)  The Guarantee Trustee shall hold office until a Successor
Guarantee Trustee shall have been appointed or until its removal or resignation.
The Guarantee Trustee may resign from office (without need for prior or
subsequent accounting) by an instrument in writing executed by the Guarantee
Trustee and delivered to the Guarantor, which resignation shall not take effect
until a Successor Guarantee Trustee has been appointed and has accepted such
appointment by instrument in writing executed by such Successor Guarantee
Trustee and delivered to the Guarantor and the resigning Guarantee Trustee.

          (d)  If no Successor Guarantee Trustee shall have been appointed and
accepted appointment as provided in this Section 4.2 within 60 days after
delivery of an instrument of removal or resignation, the Guarantee Trustee
resigning or being removed may, at the expense of the Guarantor, petition any
court of competent jurisdiction for appointment of a Successor Guarantee
Trustee. Such court may thereupon, after prescribing such notice, if any, as it
may deem proper, appoint a Successor Guarantee Trustee.

          (e)  No Guarantee Trustee shall be liable for the acts or omissions to
act of any Successor Guarantee Trustee.

          (f)  Upon termination of this Guarantee or removal or resignation of
the Guarantee Trustee pursuant to this Section 4.2, the Guarantor shall pay to
the Guarantee Trustee all amounts due to the Guarantee Trustee accrued to the
date of such termination, removal or resignation.

                                   ARTICLE V
                                   GUARANTEE

     SECTION 5.1  Guarantee
                  ---------

The Guarantor irrevocably and unconditionally agrees to pay in full to the
Holders the Guarantee Payments (without duplication of amounts theretofore paid
by the Trust), as and when due, regardless of any defense, right of set-off or
counterclaim that the Trust may have or assert. The Guarantor's obligation to
make a Guarantee Payment may be satisfied by direct payment of the required
amounts by the Guarantor to the Holders or by causing the Trust to pay such
amounts to the Holders.

     SECTION 5.2  Waiver of Notice and Demand

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<PAGE>

The Guarantor hereby waives notice of acceptance of this Guarantee and of any
liability to which it applies or may apply, presentment, demand for payment, any
right to require a proceeding first against the Trust or any other Person before
proceeding against the Guarantor, protest, notice of nonpayment, notice of
dishonor, notice of redemption and all other notices and demands.

     SECTION 5.3  Obligations Not Affected

The obligations, covenants, agreements and duties of the Guarantor under this
Guarantee shall in no way be affected or impaired by reason of the happening
from time to time of any of the following:

          (a)  the release or waiver, by operation of law or otherwise, of the
performance or observance by the Trust of any express or implied agreement,
covenant, term or condition relating to the Trust Preferred Securities to be
performed or observed by the Trust;

          (b)  the extension of time for the payment by the Trust of all or any
portion of the Distributions, Redemption Price, Liquidation Distribution or any
other sums payable under the terms of the Trust Preferred Securities or the
extension of time for the performance of any other obligation under, arising out
of, or in connection with, the Trust Preferred Securities (other than an
extension of time for payment of Distributions, Redemption Price, Liquidation
Distribution or other sum payable that results from the extension of any
interest payment period on the Debentures permitted by the Indenture);

          (c)  any failure, omission, delay or lack of diligence on the part of
the Holders to enforce, assert or exercise any right, privilege, power or remedy
conferred on the Holders pursuant to the terms of the Trust Preferred
Securities, or any action on the part of the Trust granting indulgence or
extension of any kind;

          (d)  the voluntary or involuntary liquidation, dissolution, sale of
any collateral, receivership, insolvency, bankruptcy, assignment for the benefit
of creditors, reorganization, arrangement, composition or readjustment of debt
of, or other similar proceedings affecting, the Trust or any of the assets of
the Trust;

          (e)  any invalidity of, or defect or deficiency in, the Trust
Preferred Securities;

          (f)  the settlement or compromise of any obligation guaranteed hereby
or hereby incurred; or

          (g)  any other circumstance whatsoever that might otherwise constitute
a legal or equitable discharge or defense of a guarantor, it being the intent of
this Section 5.3 that the obligations of the Guarantor with respect to the
Guarantee Payments shall be absolute and unconditional under any and all
circumstances.

There shall be no obligation of the Holders to give notice to, or obtain consent
of, the Guarantor with respect to the happening of any of the foregoing.

                                       12
<PAGE>

     SECTION 5.4  Rights of Holders

          (a)  The Holders of a Majority in liquidation amount of the Trust
Preferred Securities have the right to direct the time, method and place of
conducting any proceeding for any remedy available to the Guarantee Trustee in
respect of this Guarantee or exercising any trust or power conferred upon the
Guarantee Trustee under this Guarantee.

          (b)  Any Holder may institute a legal proceeding directly against the
Guarantor to enforce the Guarantee Trustee's rights under this Guarantee,
without first instituting a legal proceeding against the Trust, the Guarantee
Trustee or any other person or entity.

     SECTION 5.5  Guarantee of Payment

This Guarantee creates a guarantee of payment and not of collection.

     SECTION 5.6  Subrogation
                  -----------

The Guarantor shall be subrogated to all (if any) rights of the Holders against
the Trust in respect of any amounts paid to such Holders by the Guarantor under
this Guarantee; provided, however, that the Guarantor shall not (except to the
extent required by mandatory provisions of law) be entitled to enforce or
exercise any right that it may acquire by way of subrogation or any indemnity,
reimbursement or other agreement, in all cases as a result of payment under this
Guarantee, if, at the time of any such payment, any amounts are due and unpaid
under this Guarantee. If any amount shall be paid to the Guarantor in violation
of the preceding sentence, the Guarantor agrees to hold such amount in trust for
the Holders and to pay over such amount to the Holders.

     SECTION 5.7  Independent Obligations

The Guarantor acknowledges that its obligations hereunder are independent of the
obligations of the Trust with respect to the Trust Preferred Securities, and
that the Guarantor shall be liable as principal and as debtor hereunder to make
Guarantee Payments pursuant to the terms of this Guarantee notwithstanding the
occurrence of any event referred to in subsections (a) through (h), inclusive,
of Section 5.3 hereof.

                                  ARTICLE VI
                   LIMITATION OF TRANSACTIONS; SUBORDINATION

     SECTION 6.1  Limitation of Transactions

So long as any Trust Preferred Securities remain outstanding, the Guarantor
shall not (i) declare or pay any dividends or distributions on, or redeem,
purchase, acquire or make a liquidation payment with respect to, any of the
Guarantor's capital stock (which includes common and preferred stock) or (ii)
make any payment of principal of or premium, if any, or interest on or repay,
repurchase or redeem any debt securities of the Guarantor (including Other
Debentures) that rank pari passu with or junior in right of payment to the
Debentures or (iii) make any

                                       13
<PAGE>

guarantee payments with respect to any guarantee by the Guarantor of the debt
securities of any subsidiary of the Guarantor (including Other Guarantees) if
such guarantee ranks pari passu with or junior in right of payment to the
Debentures (other than (a) dividends or distributions in shares of, or options,
warrants, rights to subscribe for or purchase shares of, common stock of the
Guarantor, (b) any declaration of a dividend in connection with the
implementation of a stockholders' rights plan, or the issuance of stock under
any such plan in the future, or the redemption or repurchase of any such rights
pursuant thereto, (c) payments under the Guarantee, (d) the purchase of
fractional shares resulting from a reclassification of the Guarantor's capital
stock, (e) the exchange or the conversion of one class, or series of the
Guarantor's capital stock for another class or series of the Guarantor's capital
stock, and (f) the purchase of fractional interests in shares of the Guarantor's
capital stock pursuant to the conversion or exchange provisions of such capital
stock or the security being converted or exchanged) if at such time (1) there
shall have occurred and be continuing an event of default under the Trust
Agreement, (2) there shall have occurred and be continuing an Event of Default
under the Indenture, (3) there shall have occurred and be continuing a payment
default under the Trust Agreement or the Indenture, (4) if the Debentures are
held by the Trust, the Guarantor shall be in default with respect to its payment
of any obligations under this Guarantee, or (5) the Guarantor shall have given
notice of its election of an Extended Interest Payment Period (as defined in the
Indenture) and shall not have rescinded such notice, and such Extended Interest
Payment Period, or any extension thereof, shall have commenced.

     SECTION 6.2  Ranking
                  -------

This Guarantee will constitute an unsecured obligation of the Guarantor and will
rank (i) subordinate and junior in right of payment to Senior Indebtedness (as
defined in the Indenture, assuming the Guarantor is an "obligor" as described
therein, the term "Securities" means the Guarantee and the term "Company" means
the Guarantor), to the same extent and in the same manner that the Debentures
are subordinated to Senior Indebtedness pursuant to the Indenture, it being
understood that the terms of Article XV of the Indenture shall apply to the
obligations of the Guarantor under this Guarantee as if (x) such Article XV were
set forth herein in full and (y) such obligations were substituted for the term
"Securities" appearing in such Article XV, (ii) pari passu with the Debentures,
the Other Debentures and with the most senior preferred or preference stock now
or hereafter issued by the Guarantor and with any Other Guarantee (as defined
herein) and any guarantee now or hereafter entered into by the Guarantor in
respect of any preferred or preference stock of any Affiliate of the Guarantor,
and (iii) senior to the Guarantor's common stock.

                                  ARTICLE VII
                                  TERMINATION

     SECTION 7.1  Termination
                  -----------

This Guarantee shall terminate (i) upon full payment of the Redemption Price of
all Trust Preferred Securities, (ii) upon the full payment of the amounts
payable in accordance with the Trust Agreement, or (iii) the distribution of the
Debentures to the Holders of all of the Trust

                                       14
<PAGE>

Preferred Securities. Notwithstanding the foregoing, this Guarantee will
continue to be effective or will be reinstated, as the case may be, if at any
time any Holder must restore payment of any sums paid under the Trust Preferred
Securities or under this Guarantee.

                                 ARTICLE VIII
                COMPENSATION AND EXPENSES OF GUARANTEE TRUSTEE

The Guarantor covenants and agrees to pay to the Guarantee Trustee from time to
time, and the Guarantee Trustee shall be entitled to, such compensation as shall
be agreed to in writing between the Guarantor and the Guarantee Trustee (which
shall not be limited by any provision of law in regard to the compensation of a
trustee of an express trust), and the Guarantor will pay or reimburse the
Guarantee Trustee upon its request for all reasonable expenses, disbursements
and advances incurred or made by the Guarantee Trustee in accordance with any of
the provisions of this Guarantee (including the reasonable compensation and the
expenses and disbursements of its counsel and of all persons not regularly in
its employ) except any such expense, disbursement or advance as may arise from
its negligence or bad faith. The Guarantor also covenants to indemnify each of
the Guarantee Trustee (and its officers, agents, directors and employees) for,
and to hold it harmless against, any and all loss, damage, claim, liability or
expense including taxes (other than taxes based on the income of the Guarantee
Trustee) incurred without negligence or bad faith on the part of the Guarantee
Trustee and arising out of or in connection with the acceptance or
administration of this guarantee, including the costs and expenses of defending
itself against any claim of liability in the premises. The obligations of the
Guarantor under this Article VIII to compensate and indemnify the Guarantee
Trustee and to pay or reimburse the Guarantee Trustee for expenses,
disbursements and advances shall be secured by a lien prior to that of the Trust
Preferred Securities upon all property and funds held or collected by the
Guarantee Trustee as such, except funds held in trust for the benefit of the
holders of particular Trust Preferred Securities.

The provisions of this Article shall survive the termination of this Guarantee.

                                  ARTICLE IX
                                INDEMNIFICATION

     SECTION 9.1    Exculpation
                    -----------

          (a)  No Indemnified Person shall be liable, responsible or accountable
in damages or otherwise to the Guarantor or any Covered Person for any loss,
damage or claim incurred by reason of any act or omission performed or omitted
by such Indemnified Person in good faith in accordance with this Guarantee and
in a manner that such Indemnified Person reasonably believed to be within the
scope of the authority conferred on such Indemnified Person by this Guarantee or
by law, except that an Indemnified Person shall be liable for any such loss,
damage or claim incurred by reason of such Indemnified Person's negligence or
willful misconduct with respect to such acts or omissions.

                                       15
<PAGE>

          (b)  An Indemnified Person shall be fully protected in relying in good
faith upon the records of the Guarantor and upon such information, opinions,
reports or statements presented to the Guarantor by any Person as to matters the
Indemnified Person reasonably believes are within such other Person's
professional or expert competence and who has been selected with reasonable care
by or on behalf of the Guarantor, including information, opinions, reports or
statements as to the value and amount of the assets, liabilities, profits,
losses, or any other facts pertinent to the existence and amount of assets from
which Distributions to Holders might properly be paid.

     SECTION 9.2    Indemnification

The Guarantor agrees to indemnify each Indemnified Person for, and to hold each
Indemnified Person harmless against, any and all loss, liability, damage, claim
or expense incurred without negligence or bad faith on its part, arising out of
or in connection with the acceptance or administration of the trust or trusts
hereunder, including the costs and expenses (including reasonable legal fees and
expenses) of defending itself against, or investigating, any claim or liability
in connection with the exercise or performance of any of its powers or duties
hereunder. The obligation to indemnify as set forth in this Section 9.2 shall
survive the termination of this Guarantee.

                                   ARTICLE X
                                 MISCELLANEOUS

     SECTION 10.1   Successors and Assigns

All guarantees and agreements contained in this Guarantee shall bind the
successors, assigns, receivers, trustees and representatives of the Guarantor
and shall inure to the benefit of the Holders then outstanding.

     SECTION 10.2   Amendments
                    ----------

Except with respect to any changes that do not materially adversely affect the
rights of Holders (in which case no consent of Holders will be required), this
Guarantee may only be amended with the prior approval of the Holders of a
Majority in liquidation amount of the Securities (including the stated amount
that would be paid on redemption, liquidation or otherwise, plus accrued and
unpaid Distributions to the date upon which the voting percentages are
determined). The provisions of the Trust Agreement with respect to consents to
amendments thereof (whether at a meeting or otherwise) shall apply to the giving
of such approval.

     SECTION 10.3   Notices
                    -------

All notices provided for in this Guarantee shall be in writing, duly signed by
the party giving such notice, and shall be delivered, telecopied or mailed by
first class mail, as follows:

                                       16
<PAGE>

          (a)  If given to the Trust, in care of the Administrative Trustee at
the Trust's mailing address set forth below (or such other address as the Trust
may give notice of to the Holders and the Guarantee Trustee):

                           AGL Capital Trust III
                           c/o AGL Capital Corporation
                           2325-B Renaissance Drive
                           Las Vegas, Nevada 89119
                           Attention:  Paul R. Shlanta

          (b)  If given to the Guarantee Trustee, at the Guarantee Trustee's
mailing address set forth below (or such other address as the Guarantee Trustee
may give notice of to the Holders and the Trust):

                           BNY Trust Company of Florida, N.A.
                           Towermarc Plaza
                           10161 Centurion Parkway
                           Jacksonville, FL 32256
                           Attention:  Corporate Trust Trustee Administration

          (c)  If given to the Guarantor, at the Guarantor's mailing address set
forth below (or such other address as the Guarantor may give notice of to the
Holders of the Trust Preferred Securities and the Guarantee Trustee):

                           AGL Resources Inc.
                           817 West Peachtree Street, N.W.
                           Atlanta, Georgia 30308
                           Attention:  Richard T. O'Brien

          (d)  If given to any Holder of Trust Preferred Securities, at the
address set forth on the books and records of the Trust.

All such notices shall be deemed to have been given when received in person,
telecopied with receipt confirmed, or mailed by first class mail, postage
prepaid except that if a notice or other document is refused delivery or cannot
be delivered because of a changed address of which no notice was given, such
notice or other document shall be deemed to have been delivered on the date of
such refusal or inability to deliver.

     SECTION 10.4   Benefit
                    -------

This Guarantee is solely for the benefit of the Holders and, subject to Section
3.1(a), is not separately transferable from the Trust Preferred Securities.

     SECTION 10.5   Governing Law
                    -------------

                                       17
<PAGE>

THIS GUARANTEE SHALL BE GOVERNED BY, AND CONSTRUED AND INTERPRETED IN ACCORDANCE
WITH, THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICTS OF LAWS
PRINCIPLES THEREOF.

                                       18
<PAGE>

THIS GUARANTEE is executed as of the day and year first above written.

                                     AGL RESOURCES INC.,
                                     as Guarantor



                                     By:____________________________________
                                     Name:  Richard T. O'Brien
                                     Title: Executive Vice President and
                                             Chief Financial Officer



                                     THE BANK OF NEW YORK,
                                     as Guarantee Trustee



                                     By:____________________________________
                                     Name:
                                     Title:

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