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As filed with the Securities and Exchange Commission on April 27, 2005   SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549
 FORM 10-K/A (Amendment No. 1) þ
    
    ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
 For the fiscal year ended March 31, 2004
OR
 o
    
    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
 For the transition period from                    to
 Commission File Number 333-12430 Marconi Corporation PLC (Name of Registrant as specified in its charter)
 ENGLAND AND WALES(Jurisdiction of incorporation or organization)New Century Park
P.O. Box 53
Coventry
Warwickshire CV3 1HJ
United Kingdom(Address of principal executive offices)Not Applicable(IRS Employer identification No.) Registrants telephone number, including area code: 011 44 2476 562000. Securities registered or to be registered pursuant to Section 12(b) of the Act: None. Securities registered or to be registered pursuant to Section 12(g) of the Act: American Depositary Shares evidenced by American Depositary Receipts, each American
Depositary Share representing 2 ordinary shares, nominal value 25 pence per
share. Ordinary Shares of 25 pence per share. Securities for which there is a reporting obligation pursuant to Section 15(d) of the Act:
None. Indicate by check mark whether the registrant (1) has filed all reports required to be filed
by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or
such shorter period that the registrant was required to file such reports), and (2) has been
subject to such filing requirements for the past 90 days. Yes þ No o Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation
S-K is not contained herein, and will not be contained to the best of registrants knowledge, in
definitive proxy statements incorporated by reference in Part III of this Form 10-K or any
amendment to this Form 10-K. o Indicate by check mark if the registrant is an accelerated filer (as defined in Exchange Act
Rule 12-b-2). Yes þ No o Aggregate market value of registrants ordinary shares held by non-affiliates of the
registrant, based upon the closing price of the registrants ordinary shares on September 30, 2003, as reported by the London Stock Exchange on that date: £900,000,000. Number
of shares of registrants ordinary shares outstanding as
of April 25, 2005: 208,741,043 Ordinary Shares of 25p each.




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TABLE OF CONTENTS  Item 5: Market for Registrants Common Equity and Related Stockholder Matters:SIGNATURESEX-31.1EX-31.2    Table of Contents  EXPLANATORY NOTE We are filing this amendment to our Form 10-K for the fiscal year ended March 31, 2004, as
filed on June 14, 2004, in order to disclose an exemption that we received in the fiscal year ended
March 31, 2004 from the NASDAQ Stock Market Marketplace Rules. In accordance with Exchange Act
Rule 12b-15, we are restating Item 5, the relevant disclosure item from Form 10-K, in its entirety.
However, apart from the addition of the disclosure of our exemption from the NASDAQ Marketplace
Rules, the only changes to Item 5 are the rewording of our disclosures related to stock price
history, dividends and number of shareholders to clarify the fact that our disclosure of these
matters has not been updated since the original filing of our 10-K on June 14, 2004.




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Item 5:Market for Registrants Common Equity and Related Stockholder Matters: Stock Price History: Our ordinary shares have been listed on the London Stock Exchange since May 19, 2003. Between
that date and June 9, 2004, the highest closing market price was 732 pence and the lowest closing
market price was 270 pence. ADRs representing our ordinary shares have been quoted on the NASDAQ
National Market under the symbol MRCIY since October 2, 2003. Between that date and June 9, 2004,
the highest closing market price was 2,750 cents and the lowest closing market price was 925 cents. The following table sets forth quarterly market price information for our ordinary shares on
the London Stock Exchange (as reported by Bloomberg) and our ADRs in the over-the-counter market
and NASDAQ National Market (between October 2, 2003 and June 9, 2004) (as reported by Bloomberg). London Stock Exchange
    
    
    ADRs(2)(Sterling pence)
    
    
    (U.S. dollars)Quarterly period
    
    High
    
    
    Low
    
    
    High
    
    
    LowMay 19, 2003June 30, 2003(1)
    
    
    342.50
    
    
    
    270.00
    
    
    
    11.70
    
    
    
    9.25July 1, 2003September 30, 2003(1)
    
    
    505.00
    
    
    
    305.00
    
    
    
    16.50
    
    
    
    10.09October 1, 2003December 31, 2003
    
    
    633.00
    
    
    
    420.00
    
    
    
    22.05
    
    
    
    14.30January 1, 2004March 31, 2004
    
    
    732.00
    
    
    
    587.50
    
    
    
    27.50
    
    
    
    20.75April 1, 2004June 9, 2004
    
    
    677.50
    
    
    
    530.50
    
    
    
    24.70
    
    
    
    18.65
   (1)
    
    The pre-September 2003 market prices have been adjusted to reflect the five-for-one
share consolidation that became effective on September 9, 2003.(2)
    
    As at June 14, 2004, each ADR represented two ordinary shares, nominal value 25 pence
per share. Dividends: As at June 14, 2004, we did not have and had not in the last two fiscal years paid a dividend
on our common stock. The terms of our Senior Notes restricted us from paying a dividend until the
Senior Notes were repaid in full. Number of Shareholders: As at June 9, 2004, we had 54,163 registered holders of ordinary shares with addresses in the
United Kingdom. These U.K. ordinary shareholders collectively held 200,260,254 ordinary shares, or
99.87% of our total issued share capital as at June 9, 2004. In total, at June 9, 2004, we had
55,840 registered holders of ordinary shares who collectively held 200,511,292 ordinary shares. As at June 10, 2004, there were 56 registered holders of our ADRs. Each ADR issued represented
two ordinary shares. Of these registered ADR holders, 54 had addresses in the United States. One of
the registered ADR holders was The Depository Trust Company, which represented the total number of
ADRs held in book-entry form. The ADR holders collectively held 3,329,148 ADRs, or approximately
3.32% of our total issued share capital as at June 9, 2004. Sale of Unregistered Securities: In connection with our Financial Restructuring, on May 19, 2003, we issued to creditors having
approximately £4.8 billion of claims covered by our scheme of arrangement, 995 million ordinary
shares (199 million after our one-for-five share consolidation that occurred in September 2003),
together with the other consideration described under Item 1: BusinessFiscal 2004 Significant
DevelopmentsFinancial Restructuring, in exchange for the cancellation of their claims against us.
These ordinary shares were issued as part of a court-approved exchange pursuant to Section 3(a)(10)
of the Securities Act of 1933. Securities Authorized for issuance under Equity Compensation plans: See Item 12Security Ownership of Certain Beneficial Owners and Management.




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Share Repurchases: None Exemption from NASDAQ Corporate Governance Requirements On September 29, 2003, the NASDAQ Stock Market granted us an exemption from NASDAQ Marketplace
Rule 4350(f). Marketplace Rule 4350(f) requires a company listed on NASDAQ to provide for a quorum
for any meeting of the holders of its common stock of not less than 33 1/3% of the outstanding
shares of the companys common voting stock. By contrast, under the Companys articles of
association, two shareholders present or by proxy and entitled to vote on the business to be
transacted at a general meeting of the Company shall be a quorum.




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SIGNATURES Pursuant to the requirement of Section 13 or 15(d) of the Securities Exchange Act of 1934, the
registrant has caused this report to be signed on its behalf by the undersigned, thereunto duly
authorized. MARCONI CORPORATION PLCBy:    /s/ Michael PartonMichael PartonAuthorized SignatoryDate: April 27, 2005
 Pursuant to the requirement of the Securities Exchange Act of 1934, this report has been
signed below by the following persons on behalf of the registrant in the capacities and on the
dates indicated. Signature
    
    Capacity
    
    Date*

John Francis Devaney
    
    Chairman of the Board of Directors
    
    April 27, 2005/s/ Michael William John Parton

Michael William John Parton
    
    Director and Chief Executive Officer
(Principal Executive Officer)
    
    April 27, 2005/s/ Paviter Singh Binning

Paviter Singh Binning
    
    Director and Chief Financial Officer
(Principal Financial Officer)
    
    April 27, 2005/s/ Christopher Charles Holden

Christopher Charles Holden
    
    Group Financial Controller
(Principal Accounting Officer)
    
    April 27, 2005*

Michael Kent Atkinson
    
    Director
    
    April 27, 2005*

Michael John Donovan
    
    Director
    
    April 27, 2005*

Kathleen Ruth Flaherty
    
    Director
    
    April 27, 2005*

Peter Charles Fletcher Hickson
    
    Director
    
    April 27, 2005*

Werner Karl Koepf
    
    Director
    
    April 27, 2005*

Douglas Francis McWilliams
    
    Director
    
    April 27, 2005
  *Signed under power of attorney by Michael Parton.