OTTAWA - The head of Elections Canada says he's received no evidence to back Conservative claims of illegal advertising at the NDP's national convention last June.

"The only thing I've seen so far is a letter from the party making some allegations about possible illegal contributions," chief electoral officer Marc Mayrand told a Commons committee today.

"There's no evidence with it."

Mayrand said the matter has been referred the commissioner of elections and he is "looking into it."

Last week, the ethics commissioner told MPs there are no "reasonable grounds" for her to investigate a Tory ethics complaint over union sponsorships at the NDP convention.

Mayrand refused to comment on the specific allegations, but he said advertising at party conventions is OK as long as a legitimate market exists and the amount paid represents fair market value.

New Democrat MP Charlie Angus said its "interesting" the Tory claims are before the Commons ethics committee the same week the Conservatives' so-called "in-and-out" election financing scheme is before the courts.

The Tories say signs were posted during the NDP's policy convention last June that indicated unions were sponsoring various events.

The Canada Elections Act prohibits unions from making political contributions.

In an Aug. 31 letter to Elections Canada, Conservative party lawyer Arthur Hamilton said "it appears the NDP has received what the commissioner of Elections Canada has deemed to be contributions in contravention of the Elections Act."

"The Conservative Party of Canada would therefore request that your office take the steps it deems necessary to review this matter fully and identify any contributions which must be returned."

Hamilton attached several photos taken of signs and flyers from the convention.

One of the events that included a union sponsorship was a dinner featuring the late NDP leader Jack Layton and Nova Scotia Premier Darrell Dexter. The sign for the event carried the symbol of the Public Service Alliance of Canada.

A flyer for another $300-a-ticket "intimate reception" with Layton featured the logo of the United Steelworkers union.

Heather Wilson, the NDP's director of fundraising and membership, has said advertisements and sponsorships at "fair market value" are allowed by law and Elections Canada is aware of the practice.

"If there is indeed a market and the possibility of a genuine transaction, the second question is whether the amount paid for the advertising space represents fair market value," Mayrand told MPs.

"If the amount received is greater than the value of any service provided, then a contribution will have occurred. Determinations regarding the existence of a market and fair market value of a good and service are essentially questions of fact that involve careful consideration of all the circumstances of a particular transaction."

Earlier this year, four Conservative officials were charged with regulatory violations of the Canada Elections Act related to electoral overspending.

Under the in-and-out scheme, the party transferred funds to dozens of riding associations and then directed them to send the money right back in order to pay for radio and TV advertising during the 2006 election.

Elections Canada argued the advertising was national in nature and resulted in the party exceeding its spending limits. The Conservatives disagree, and have been fighting Elections Canada in court.

The Federal Court of Appeals sided with Elections Canada and the Tories have filed an appeal with the Supreme Court of Canada.

The Conservative claim against the NDP generated an indignant response from the United Food and Commercial Workers Union.

The unions top Canadian official issued a news release calling it "a groundless, strategic assault on the labour movement" timed to coincide with Labour Day.