Canadian leader Justin Trudeau's ex-right hand man defended him in parliament on Wednesday over accusations of meddling in the prosecution of a corporate giant that have plunged the prime minister into his worst crisis in office.

Trudeau's Liberal government has been rocked for weeks by the snowballing scandal, which has triggered the resignation of two cabinet ministers and badly tarnished the image of the golden boy premier.

The blow to Trudeau's reputation at home and abroad could be deeply damaging for the Canadian leader, who, since being elected in 2015, had been considered one of the most likeable and trustworthy leaders on the world stage, especially by young liberals who praised his progressive views on, among other things, feminism, climate change and immigration.

(When asked why he had made it a priority to have a half-female, half-male cabinet when he formed his government, he famously replied "Because it's 2015").

WHAT'S THE SCANDAL ABOUT?

Gerry Butts, an architect of Trudeau's 2015 victory and a longtime friend, quit as his principal secretary last month, saying he did not want to become a distraction as the government braces for a tough general election in eight months.

Testifying before the House of Commons justice committee on Wednesday, Butts rebutted accusations made by former Attorney-General Jody Wilson-Raybould, whose abrupt resignation last month touched off the crisis.

"I am firmly convinced that nothing happened here beyond the normal operations of government," Butts said.

"If anyone crossed a line, it was the responsibility of the minister to inform somebody about that. And I'm presenting myself as the most likely person that would have been informed, and I was not."

The scandal centres on allegations that Trudeau's inner circle intervened to shield Canadian engineering giant SNC-Lavalin from a bribery trial.

The Montreal-based firm faces corruption charges for allegedly paying tens of millions of dollars in bribes between 2001 and 2011 to secure Libyan government contracts under the rule of Moamer Kadhafi.

SNC-Lavalin openly lobbied the Canadian government for an out-of-court settlement that would mean paying a fine and agreeing to compliance measures.

A conviction, the company argued, risked crippling its business and putting thousands out of work.

Wilson-Raybould refused to ask prosecutors to settle, and the trial is set to proceed.

Since leaving office, she has told politicians she experienced "consistent and sustained" pressure from Trudeau's inner circle to interfere in the case, including "veiled threats." Trudeau denies the allegations, saying he made clear to Wilson-Raybould that any decision on the case was hers alone.

"When you boil it all down, all we ever asked the Attorney-General to do was to consider a second opinion. When 9,000 people's jobs are at stake, it is a public policy problem," Butts said.

"It was our obligation to exhaustively consider options the law allows … to demonstrate that (the decision) was taken with great care and careful consideration of their livelihood."

HAS THE SCANDAL REALLY HURT TRUDEAU?

Support for the beleaguered prime minister and his Liberals has fallen for the first time behind the opposition Tories, a new poll showed on Tuesday, a day after Treasury Board president Jane Philpott became the second cabinet minister to break ranks with Trudeau and quit in protest.

With opposition leaders clamouring for Trudeau to resign — supported by half of Canadians, according to one recent survey — the justice committee and the independent ethics commissioner have both opened investigations.

In testimony before the committee, Wilson-Raybould named 11 government officials, including Butts, saying they "urged me to take partisan political considerations into account, which was clearly improper."

She claims they continued to "hound" her from September to December after she made known her decision not to offer SNC-Lavalin a deal.

That pressure was not illegal, she said, but "raised serious red flags in my view."

Butts said there had been a total of 10 meetings and 10 calls on the matter, including only one face-to-face discussion in December between himself and Wilson-Raybound.

"I do not see how our brief discussion of that file constituted pressure of any kind," he said.

He disputed the contention of Wilson-Raybould, Canada's first indigenous Attorney-General and justice minister, that she was moved to a new portfolio over her refusal to take action in the case.

SNC-Lavalin, its international arm and another subsidiary are accused of having offered Can$47 million (US$36 million) in bribes to Libyan officials 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 freshwater to the cities of Tripoli, Benghazi and Sirte.

WHAT'S NEXT?

Canada's ethics commissioner is investigating the affair but that will take months.

Andrew Scheer, leader of the opposition Conservative Party, has called on Trudeau to resign and is asking police to investigate.

Trudeau's Liberal party has the majority of seats in Parliament so his government cannot be brought down in a vote of no confidence unless his own party members vote against him. Federal elections are being held in October, however, and at least one poll says the Liberals are now trailing the Conservatives by a small margin.

If Trudeau should lose another Cabinet minister before the election his leadership could be called into question and already shaken Liberals could rebel. So far, other Liberal Cabinet ministers are rallying around him.

Time will tell how the scandal influences the country's perception of their leader, and if those who voted for Trudeau in the last election turn against him when Canada go to the polls in October.