Canada's Prime Minister Justin Trudeau apologized Monday for not recusing himself from discussions about a government contract to a charity that had paid his family large sums of money.

"I made a mistake not recusing myself immediately from the discussions, given our family's history. And I'm sincerely sorry," he told a news conference.

His apology came amid opposition demands for an investigation into a $900 million contract awarded by the government to development and youth advocacy organization WE Charity.

The NGO has admitted paying nearly CAN $300,000 (US$220,000) to Trudeau's mother, brother and wife for speaking engagements.

And Trudeau himself admitted to taking part in negotiations with the charity over the contract.

He said he knew his mother Margaret worked for WE as an advocate on mental health issues. 

"I didn't know the details of how much she was getting paid by various organizations, but I should have and I deeply regret that," he said.

On Sunday, Canada's opposition Conservative Party said it would ask Trudeau to testify under oath before the House of Commons finance committee about the contract. 

"Either the PM can agree voluntarily to show up and attend -- that's what he will do if he has nothing to hide -- or Parliament can compel him to appear," Conservative lawmaker Pierre Poilievre said.