Frank Dunn, the deposed chief executive officer of Nortel Networks, sued the company for wrongful dismissal last month, Nortel revealed today in regulatory filings.

Dunn was fired in April 2004 amid an investigation of accounting impropriety that still plagues the company today. An independent investigation commissioned by Nortel and published in January 2005 maintained that Dunn took advantage of accounting errors to report nonexistent profits in 2003, triggering the payment of executive bonuses.

Dunn filed suit against Nortel in the Ontario Superior Court of Justice on April 20, 2006, accusing Nortel of wrongfully dismissing him, defaming him and causing him mental distress. The suit seeks unspecified punitive, exemplary and aggravated damages.

On April 24, the court granted Dunn’s request to seal the suit’s file while it views his motion to stay his action and seal the file.

Early last year, Nortel filed a civil suit against Dunn--as well as former officers Douglas Beatty and Michael Gollogly--in the same court, seeking the return of payments made to them under the executive bonus plan that Nortel has said those officers manipulated.

Last month media outlets reported that Dunn asked the court in March for an order freeing him from testifying in the Ontario Securities Commission’s ongoing investigation of Nortel. According to Bloomberg News, Dunn’s application argued that any testimony he might give to the OSC could be used against him in a hypothetical U.S. criminal proceeding. (The U.S. Securities and Exchange Commission is also currently investigating Nortel’s accounting misdeeds.) However, the court denied Dunn his order, requiring him to testify to the OSC.