The international economic group that oversees global anti-bribery cases has expressed concern over allegations that Canadian Prime Minister Justin Trudeau interfered in a criminal prosecution against a top engineering firm accused of bribing Libyan officials between 2001 and 2011.

Canada is one of 44 nations that in 1999 signed the legally binding Anti-Bribery Convention, which established international standards to criminalise the bribery of foreign officials.

The statement issued by the working group of OECD said it had already written to the prime minister's office and was keeping a close eye on investigations by the House of Commons justice committee and the federal ethics commissioner.

It said Canada has pledged to update the group on the matter at the working group's meeting to be held in June.

Opposition lawmakers accuse Trudeau of trying to cover up an attempt by officials to help the SNC-Lavalin, which could be banned from bidding for federal contracts for a decade if found guilty.

The scandal has already cost Trudeau his closest adviser Gerald Butts who resigned last month but denied he had or others had improperly pressured Wilson-Raybould.

Trudeau has rejected calls to resign over the scandal, saying he and his staff always acted properly and that Canadians will get to have their say on the matter at the federal election in October.

Last year, the Liberals introduced a new law to allow the director of public prosecutions to use what are known internationally as deferred-prosecution agreements (DPAs).

These agreements shelve a criminal prosecution of a company in exchange for the company admitting wrongdoing, paying fines, giving up any money it earned in the commission of the crime it is accused of and agreeing to be monitored for a period of time.

If the company fulfils the terms of the agreement, the criminal penalties can be dropped. If the company fails to keep its end of the bargain, the charges can still go forward.

Last fall, the director of public prosecutions in Canada decided SNC-Lavalin was not eligible for a remediation agreement. Wilson-Raybould, then the attorney general, decided not to use her authority to overrule that decision.

Wilson-Raybould said that after that, multiple people from Trudeau's office, the finance minister's office and the Privy Council Office all put sustained, improper pressure on her to change her mind.

She said that when she wouldn't do so, she was shuffled out as minister of justice and attorney general to the lower-profile job of minister of veterans affairs. She ultimately resigned from cabinet entirely.

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