OTTAWA—Canada’s ethics watchdog ruled that Prime Minister Justin Trudeau broke conflict-of-interest laws by trying to steer the then-attorney general away from criminally prosecuting a Montreal company.

The findings from Canada’s Office of the Conflict of Interest and Ethics Commissioner on Wednesday represent a fresh blow to Mr. Trudeau and his incumbent Liberal government, weeks before his re-election campaign is set to start toward an Oct. 21 vote. The report presents new details about a political crisis that gripped Canada through the winter and spring, particularly on how representatives for SNC-Lavalin Group Inc. put pressure on Mr. Trudeau’s Liberal government.

The Liberals had recently begun to recover in the polls after public support for Mr. 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. Ms. Wilson-Raybould backed the prosecutors’ decision after seeking advice from her aides, justice department officials, and former Canadian attorneys-general.

Mr. 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. Ms. Wilson-Raybould stepped down from cabinet in February after the allegations became public.