Thomas Mulcair. (ERIC BOLTE/QMI Agency)

OTTAWA - The federal Conservatives called on the NDP Wednesday to come clean on how much donation cash has been returned to its union friends after Elections Canada ruled the official Opposition broke the law.

Elections Canada says the NDP violated Section 404 of the Elections Canada Act when it accepted money from unions to sponsor its 2011 national convention.

The NDP won't say how much money it received, nor from whom and whether any has been paid back.

Fred DeLorey, a Conservative party spokesman, said the sum of "potentially thousands upon thousands of dollars given to the party by union bosses and paid for by union dues, are unquestionably illegal donations and that money collected must be paid back."

NDP Leader Thomas Mulcair appeared to suggest Wednesday no rules were broken a day after Elections Canada said otherwise and ordered the party to return the money within 30 days as the law states or face a fine.

"We continue as the letter says to be in conformity with the law. We continue to obey the law. That's what Elections Canada confirms," he said.

This is the second time in a year the NDP has run afoul of Elections Canada financing laws.

It was caught red-handed at the time of Jack Layton's death when it set up a memorial page on the party's website and invited mourners to send money to a newly minted left-wing think-tank with the promise of a tax receipt.

Mark.Dunn@sunmedia.ca

Twitter: @MarkDunnSun