NDP Leader Tom Mulcair says Prime Minister Stephen Harper needs to "come clean" over the deletion of an email account belonging to a former PMO staffer involved in negotiations over the repayment of Senator Mike Duffy's ineligible expenses.

"Stephen Harper's employees, for whom he is responsible, have illegally erased documents that had an important involvement in an ongoing investigation and Prime Minister Harper has to start explaining himself," Mulcair told reporters in a press conference across from Parliament Hill Wednesday.

NDP Leader Tom Mulcair

NDP Leader Tom Mulcair is calling for an investigation after officials working for Prime Minister Stephen Harper deleted an email account, despite signs the account has been recovered and turned over to the RCMP as part of its Mike Duffy probe. (Chris Wattie/Reuters)

The emails were later recovered during the course of an RCMP investigation into a payment to Duffy from Harper's former chief of staff, Nigel Wright.

Muclair was reacting to a news report that said memos to IT staff in the Privy Council Office (PCO), the non-partisan administrative arm of the Prime Minister's Office, had instructed them not to delete the accounts of departing staff.

Nevertheless, days after the memos were sent, an account belonging to Benjamin Perrin was apparently deleted when he left his job in late March 2013. Perrin was legal counsel to the prime minister and had been involved in discussions within the PMO on how to deal with Duffy's expenses.

Until then, it had been policy for departing staff to ensure emails for ongoing files were forwarded to the appropriate officials or put in electronic storage.

Emails recovered

The account deletion first became public in November 2013, when court documents about the probe were made public.

But days after the court documents were released, PCO officials announced they hadn't deleted the emails: in fact, Perrin's account had been frozen "due to unrelated litigation."

New court records obtained by CBC News suggest the information in Perrin's account may have been provided to the RCMP in January 2014.

Despite the apparent recovery of the emails, Mulcair called Tuesday for an investigation into the account deletion.

"Canadians have a right to know what went on and destroying that type of evidence as they've done by deleting the emails simply puts another circle around that big stain of what the Conservatives have been doing: hiding information, trying to block Canadians' understanding of the big lie that they constructed with Mike Duffy.

"These are emails that would have gone right to the heart of this matter and that's why they erased them," Mulcair said.

'Completely false'

A spokesman for Harper said Mulcair's allegations are serious but "without a shred of evidence," and said there was no new information in the news report Tuesday.

"His suggestion that anyone in the PMO illegally destroyed emails is unsubstantiated and completely false," Jason MacDonald said in an email to CBC News.

"The RCMP has been provided whatever assistance and documents they required for their investigation. Specifically, all documents requested by the RCMP have been provided to them. The issue regarding Mr. Perrin’s emails was resolved months ago, when PCO discovered they in fact still had the emails in question, and they were provided to the RCMP."

The court records released in November contain numerous excerpts from emails allegedly sent by PMO staff, including Perrin and Wright, regarding negotiations with Duffy to cover the repayment of his Senate living expenses.

Mulcair's allegation that the emails were deleted illegally is based on rules set out in the Library of Parliament Act, which state most ministerial office records must be retained.

"They know the rules of Library of Parliament, they know the rules of archives. They're breaking the law by erasing these documents, " Mulcair said, comparing the situation to allegations against the Ontario Liberals over a decision to cancel construction of a gas plant during the last provincial election campaign.

"It's not only wrong, it's illegal."