The scandal, revealed earlier this year, tarnished the prime minister's golden boy image, and cost two ministers and two senior officials their jobs, while support plunged for his Liberals before they clawed back some ground. New polls show the party in a dead heat with the opposition Conservatives.
Independent parliamentary ethics commissioner Mario Dion said Trudeau and his officials had wrongly sought to "exert influence over the attorney general in her decision whether to intervene in a matter relating to a criminal prosecution."
It marks the second time that Trudeau has been found in breach of Canada's ethics laws, after being rebuked in 2017 for accepting a paid family vacation on the private island in the Bahamas of the Aga Khan, a business magnate and spiritual leader of Ismaili Muslims.
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Trudeau must pay a small fine of up to Can$500 (US$375) for contravening Canada's conflict of interest act, but with only two months before national elections the political costs could be much steeper.
SNC-Lavalin, one of the world's major engineering firms, was charged in 2015 with allegedly paying Can$47 million in bribes between 2001 and 2011 to secure contracts in Libya during the rule of former strongman Moamer Kadhafi, and of defrauding the Libyan government of Can$130 million.
The charges relate to the world's largest irrigation scheme - the Great Man-Made River Project - to provide fresh water to the cities of Tripoli, Benghazi and Sirte.
An opposition party leader, Jagmeet Singh, called the ethics breach "outrageous," while Conservative Party chief Andrew Scheer said it was "an unforgivable assault on the independence of our justice system."
"Canadians understand we have to be vigilant against those who want to abuse the power of their office and engage in this type of corrupt behavior and I do believe this will be top of mind in this election," Scheer added.
Trudeau has steadfastly denied accusations that his inner circle sought to shield SNC-Lavalin from a corruption trial.
On Wednesday, he offered a nuanced mea culpa.
"I disagree with some of (the commissioner's) conclusions but I fully accept this report and take responsibility for everything that happened," he said.
"But at the same time, I can't apologize for standing up for Canadian jobs, because that's part of what Canadians expect me to do."
Earlier his office released an independent review of the joint roles of Canada's attorney general and justice minister, rejecting calls to split the two jobs.
Attorney general Jody Wilson-Raybould refused to ask prosecutors to settle the case, and the trial is set to proceed.
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But after resigning, Canada's first indigenous attorney general testified to lawmakers that she had experienced "consistent and sustained" political pressure to interfere in the case, including "veiled threats."
Dion concluded: "The authority of the prime minister and his office was used to circumvent, undermine and ultimately attempt to discredit the decision of the director of public prosecutions as well as the authority of Ms Wilson-Raybould as the Crown's chief law officer."
He said Trudeau "directed his staff to find a solution that would safeguard SNC-Lavalin's business interests in Canada."
He also found that "partisan political interests were improperly put to the attorney general for consideration in the matter."
A conviction at trial would result in SNC-Lavalin being deprived of lucrative government contracts resulting in up to 9,000 jobs lost, according to the company.
The Montreal-based firm openly lobbied the government for an out-of-court settlement that would result in a fine and agreeing to compliance measures.
Wilson-Raybould's replacement as attorney general, David Lametti, has so far only said he is considering the issue.
In April, Trudeau kicked Wilson-Raybould and budget minister Jane Philpott out of the Liberal Party. Philpott had resigned her cabinet post in solidarity with Wilson-Raybould and criticized Trudeau's handling of the case.
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