Martin referred to a press conference he held on February 23 in which he expressed his "personal outrage" about the revelation that Elections Canada was investigating misleading robocalls in Guelph, Ontario.

"I singled out a private individual, Mr. Matt Meier, along with his business RackNine Inc. and I wrongfully accused them of being part of a conspiracy to commit electoral fraud," he said.

"I now know that the statements I made insinuating Mr. Meier’s and RackNine’s participation in an electoral fraud conspiracy were wholly and unequivocally false.... To my knowledge, neither Mr. Meier, nor RackNine, including any employees of RackNine, has ever been investigated for involvement in electoral fraud in the 2011 general election or otherwise."

"Racknine was merely an innocent intermediary, not a participant, in electoral fraud," Martin said Monday, adding that he was authorized by the NDP to apologize for similar damage publications on the party's website may have caused.

Meier's lawyer said in a series of emails that the apology is long overdue, but that it doesn't satisfy Martin's legal requirements.

"My client continues to try to piece together his life and his business after these harmful acts of defamation," Justin Matthews told CBC News.

'Court action will continue'

"Defamation is easily done yet difficult to repair. My clients have been damaged and it remains to be seen if those damages can be corrected."

"The court action will continue until such time that the damages my clients have suffered are recognized and repaired," Matthews said.

Martin is one of the party's lead critics on fraudulent election calls. He has previously had to clarify comments he made about Campaign Research, a company hired by the Conservative Party to do voter identification and get-out-the-vote calls. He also had to apologize to RMG, another Conservative-linked firm that does similar work.

Racknine had threatened to sue Martin for $5 million over the comments.

Little new information has emerged on the issue since Chief Electoral Officer Marc Mayrand spoke to a House committee about the investigation at the end of March.

Mayrand told MPs the election agency is investigating 800 complaints about robocalls and live calls in 200 ridings from lead-up to the May 2, 2011 federal vote.

An Elections Canada investigator has traced the calls, which illegally claimed to be on behalf of the agency and sent voters to the wrong polling station, to a phone bought by someone using the pseudonym Pierre Poutine. Information that's been made public so far hasn't pointed to a perpetrator.