In announcing her decision, Ms. Philpott cited accusations that Mr. Trudeau and his aides had exerted improper and excessive pressure on the justice minister and attorney general at the time, Jody Wilson-Raybould, to seek a settlement in the criminal case against the company, SNC-Lavalin.

A criminal conviction would have barred the company from bidding on government contracts for a decade, potentially leading to major job losses in Quebec, where it has its headquarters.

[You can read more here about the SNC-Lavalin case and how it has entangled Mr. Trudeau.]

Ms. Wilson-Raybould resisted the pressure, and quit the cabinet last month. Ms. Philpott was among the few cabinet ministers to publicly side with her.

“There can be a cost to acting on one’s principles, but there is a bigger cost to abandoning them,” Ms. Philpott wrote in her resignation letter to Mr. Trudeau on Monday.

Privately, several members of his caucus have said Mr. Trudeau has badly fumbled his handling of the controversy. Publicly, though, they have spoken in solidarity with the prime minister.

Royce Koop, a political scientist at the University of Manitoba in Winnipeg, said that might now change.

“Philpott was really the talent pool of the Trudeau cabinet, she got things done,” he said. “The question now is do we now start to see the beginning of a caucus rebellion? I haven’t heard that anything is happening. But this is going to shift opinion in the caucus.”