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

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

His apology came amid opposition demands for an investigation into a CAN$900 million (USD $662 million) contract awarded by the government to development and youth advocacy organization WE Charity.

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

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 but didn't know the "details."

"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.

The episode is already under investigation by the country's ethics watchdog, and marks the third time in as many years that Trudeau has found himself at the center of an ethics firestorm.

The ethics commissioner has already released two reports concluding that Trudeau violated the Conflict of Interest Act: in 2017, for taking a vacation on the Aga Khan's private island, and last year for trying to influence legal proceedings in the SNC-Lavalin case, in which a subsidiary of the engineering company was accused of paying bribes to secure contracts in Libya.

WE Charity has said it was withdrawing from the $900 million federal program, which provides grants of up to $5,000 to eligible students for volunteer work with non-profit organizations during the coronavirus pandemic.

It had hired more than 450 contract workers, according to the Toronto Star, and is now forced to lay them off.

Trudeau said he was "really disappointed in myself," adding: "I think Canadians will make their judgments about what we were trying to do and what I should have done."

"But at the same time, we're going to keep focusing on delivering for Canadians so we can get through this pandemic, come back with a stronger economy, while at the same time, we make sure that vulnerable Canadians and young people have greater opportunities than they had before," he said.