Exhibit 12.1

                                 CERTIFICATION

I, Donald W. Smith, certify that:

1.    I have reviewed this annual report on Form 20-F of Mitel Networks
      Corporation;

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 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 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 this 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 report 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.

5.    The registrant's other certifying officers and I have disclosed, based on
      our most recent evaluation of internal control over financial reporting,
      to the registrant's auditors and the audit committee of 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 control 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 control
      over financial reporting.

Date: July 31, 2003

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