Liberals are demanding an audit of controversial expenses claimed by Natural Resources Minister Lisa Raitt during her stint as president of the Toronto Port Authority.

Paul Szabo, the Liberal chair of the House of Commons ethics committee, added Auditor General Sheila Fraser on Thursday to the list of independent parliamentary watchdogs he's asked to investigate alleged ethical lapses by Raitt.

His latest request for an audit followed a Toronto Star report alleging that Raitt signed off on her own expenses at least once - for a $3,000 trip to London, England - during her term as port president.

Toronto lawyer Michele McCarthy, a former chair of the port's board of directors, told the newspaper she refused to sign off on the trip, which was not pre-approved, so Raitt signed it herself.

The Star also obtained documents which suggest Raitt racked up expenses of almost $130,000 and signed off on a number of claims. In other cases - including a $9,000 lunch for 50 people at a Toronto steakhouse - no one appears to have approved the claim.

McCarthy said all Raitt's expenses should have been approved by the board chair.

McCarthy is one of four former directors who, earlier this year, asked Fraser to investigate spending irregularities at the port authority. Fraser said she had no mandate to launch a special investigation.

In a letter Thursday to Stephen Harper, Szabo asked the prime minister to grant Fraser a mandate to conduct a special audit of the port authority's operations during Raitt's tenure. The authority is an arms-length organization funded by the federal government.

In the Commons, Szabo accused Raitt of unethical conduct and the government of executing a "shameful coverup" of the affair.

Raitt did not respond, leaving her defence to Transport Minister John Baird. He maintained the port authority has already determined that "all expense and hospitality policies were followed" and that "there was nothing unusual in expenses for a business of this size".

The parliamentary ethics and privacy commissioners are already conducting investigations into a separate complaint by Szabo and the NDP that Raitt used the port authority's email list and office to organize a political fundraiser last September.

Szabo has also asked Elections Canada and the lobbying watchdog to look into that affair.