The Liberals had recently begun to recover in the polls after public support for  Trudeau tumbled in the wake of revelations in February that the Canadian leader and his senior officials had tried to persuade the attorney-general, Jody Wilson-Raybould, to let the engineering-and-construction firm avoid a criminal trial on bribery and fraud charges.

Federal prosecutors last fall denied a request by SNC-Lavalin to avoid a trial and negotiate an out-of-court settlement. Wilson-Raybould backed the prosecutors’ decision after seeking advice from her aides, justice department officials, and former Canadian attorneys-general.

Trudeau and senior officials in his office pressed her and her chief aide on multiple occasions in calls and messages as well as in-person to reconsider the decision to prosecute, arguing that it could put thousands of jobs across Canada at stake, the report said. She resisted and earlier this year was moved to another cabinet post. Wilson-Raybould stepped down from the cabinet in February after the allegations became public.

“The authority of the prime minister and his office was used to circumvent, undermine and ultimately attempt to discredit the decision of [federal prosecutors] as well as the authority of Wilson-Raybould,” said Mario Dion, the ethics commissioner. “I find all of these tactics troubling.”

Dion’s report said the repeated interventions by Trudeau and his aides despite the attorney general’s steadfast refusal to intervene “lead me to conclude that these actions were tantamount to political direction.”

The ethics watchdog said Mr. Trudeau violated a section of Canada’s conflict-of-interests law that prohibits public-office holders from using their posts to improperly further someone else’s private interests. SNC-Lavalin had significant financial interests to avoid a trial, the report found.

A spokeswoman for Dion’s office said Canada’s conflict-of-interest law doesn’t give the ethics watchdog the power to impose sanctions on a public-office holder once violations are discovered through a probe.

Trudeau, speaking to reporters in Niagara-on-the-Lake, Ontario, said he disagreed with some of the ethics watchdog’s conclusions. “I can’t apologize for standing up for Canadian jobs,” he said. “We need to be able to talk about the impacts on Canadians of decisions being made.”

He said he nevertheless accepted responsibility for what unfolded. “This was a situation that should not have happened,” said Trudeau. “I take full responsibility.”

In a statement, Ms. Wilson-Raybould said the ethics probe’s findings confirm critical facts she has shared with Canadians and affirm her view that federal prosecutors need to act independently. “The report reminds us that we must all remain vigilant,” she said.

Wilson-Raybould is running for Parliament in Vancouver as an independent in this fall’s election.

The watchdog’s report marks the second time Trudeau has been found to have contravened ethics laws. In late 2017, the same office ruled the prime minister violated guidelines when he accepted an invitation to a Bahamian island owned by the Aga Khan, the spiritual leader of Shia Ismaili Muslims.

Trudeau’s political opponents said the report was evidence that voters need to oust the Liberal government on Oct. 21.

“We have a clear picture of who Trudeau is,” said Andrew Scheer, leader of the Conservative Party, which in several recent polls either leads or is in a statistical tie with the Liberal Party. Trudeau, he said, “has used the power of his office to enrich himself, reward his friends and punish his critics.”

The report threatens Trudeau’s re-election chances, one political analyst said, by redirecting the spotlight to a political scandal that has damaged the Liberal Party brand.

“These are not political accusations. This is an independent officer of Parliament coming out saying: ‘You have fundamentally interfered in the rule of law.’ This is really serious,” said Duane Bratt, a politics professor at Mount Royal University in Calgary, Alberta.

The report also revealed how closely SNC-Lavalin and the Liberal administration interacted in trying to avert a criminal trial. Company executives pressed senior Canadian officials in early 2018 to introduce changes to allow companies like SNC-Lavalin to negotiate out-of-court settlements. Over a month later, the budget plan incorporated amendments along those lines.

Dion’s report said SNC-Lavalin representatives and lawyers stepped up communications with Trudeau’s office in November of last year about finding a way to override  Wilson-Raybould’s decision not to intervene. Around the same time, the report said, lawyers for the company were circulating legal opinions to other members of the Liberal cabinet suggesting there were legitimate reasons for the attorney-general to intervene.