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                                                                    EXHIBIT 12.1

                            SECTION 302 CERTIFICATION

I, Donald W. Smith, certify that:

1.    I have reviewed this annual report on Form 20F of Mitel Networks
      Corporation for the period ended April 24, 2005, and for the six (6) day
      transition period ended April 30, 2005;

2.    Based on my knowledge, this annual report does not contain any untrue
      statement of a material fact or omit to state a material fact necessary in
      order to make the statements made, in light of the circumstances under
      which such statements were made, not misleading with respect to the period
      covered by this annual report;

3.    Based on my knowledge, the financial statements, and other financial
      information included in this annual report, fairly present in all material
      respects the financial condition, results of operations and cash flows of
      the registrant as of, and for, the periods presented in this annual
      report;

4.    The registrant's other certifying officers and I are responsible for
      establishing and maintaining disclosure controls and procedures (as
      defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant
      and we have:

      a)    designed such disclosure controls and procedures, or caused such
            disclosure controls and procedures to be designed under our
            supervision, to ensure that material information relating to the
            registrant, including its consolidated subsidiaries, is made known
            to us by others within those entities, particularly during the
            period in which the annual report is being prepared;

      b)    evaluated the effectiveness of the registrant's disclosure controls
            and procedures and presented in this annual report our conclusions
            about the effectiveness of the disclosure controls and procedures,
            as of the end of the period covered by this annual report based on
            such evaluation; and

      c)    disclosed in this annual any change in the registrant's internal
            control over financial reporting that occurred during the period
            covered by this annual report that has materially affected, or is
            reasonably likely to materially affect, the registrant's internal
            control over financial reporting; and

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5.    The registrant's other certifying officers and I have disclosed, based on
      our most recent evaluation of internal controls over financial reporting,
      to the registrant's auditors and the audit committee of the registrant's
      board of directors (or persons performing the equivalent function):

      a)    all significant deficiencies and material weaknesses in the design
            or operation of internal controls over financial reporting which are
            reasonably likely to adversely affect the registrant's ability to
            record, process, summarize and report financial information; and

      b)    any fraud, whether or not material, that involves management or
            other employees who have a significant role in the registrant's
            internal controls.

Date: October 20, 2005


                                     /s/ Donald W. Smith
                                     -----------------------------------
                                     Name: Donald W. Smith
                                     Title:  Chief Executive Officer