A former senior aide to Prime Minister Justin Trudeau has left the Prime Minister's Office to work for a major law firm, where he will help clients deal with Canada's "nuanced regulatory landscape."

Cyrus Reporter was chief of staff to Mr. Trudeau when the Liberals were in opposition between 2013 and 2015, and then a senior adviser in the PMO. In that role, he was specifically tasked with handling complex and controversial files.

The 49-year-old, who is a father of a 15-month-old baby and a teenager, has become a partner at Gowling WLG and will be based in Ottawa.

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While he acted as a registered lobbyist before 2013, Mr. Reporter will be banned from lobbying at the federal level for five years. Still, he will be able to advise clients on the best possible way for them to deal with the Canadian government and advance their interests in Ottawa.

He also plans to stay involved with the Liberal Party of Canada as a volunteer and could once again be involved in the next general election for Mr. Trudeau's team. Mr. Reporter was a constant presence alongside Mr. Trudeau and his other top advisers during the 2015 election campaign.

He is the senior-most official in the PMO to quit since the Liberals formed government 16 months ago.

"The four-plus years that I have spent in Mr. Trudeau's employ have been the experience of a lifetime, and I will cherish it forever," Mr. Reporter said in an interview. "It's simply a time for me now to focus on family life and the next stage of my career."

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Between 2003 and 2013, Mr. Reporter worked as a lawyer with Fraser Milner Casgrain. In that position, he acted as a registered lobbyist for a number of clients, including Brazilian aerospace giant Embraer SA, AltaGas Ltd., BP Canada Energy and Nexen Inc.

He oversaw the Liberal team in Parliament after Mr. Trudeau became the party's leader in 2013. In government, Mr. Reporter was regularly put in charge of the hottest files of the day, including dealing with the Ethics Commissioner on the investigation into Mr. Trudeau's trip to a secluded island in the Bahamas, and fulfilling the 2015 promise to welcome 25,000 Syrian refugees.

The long-time political adviser was also involved in plans to legalize marijuana and medical assistance in dying, as well as those to replace Canada's fleet of CF-18s. For a short time, he also did double duty as interim chief of staff in the office of Justice Minister Jody Wilson-Raybould.

As a former designated public-office holder, he will be banned from lobbying the federal government for five years. He will also be prevented from working for the next year on files in which he was directly involved in government, such as the legalization of marijuana.

At Gowling, Mr. Reporter will specialize in the fields of public policy and regulatory and ethics law. He can be expected to help companies coming into Canada to meet all of their obligations under the country's lobbying and conflict-of-interest regimes, for example.

The Ethics Commissioner's website shows that Mr. Reporter officially left the government on Feb. 6. He advised the Commissioner last month that he had received a formal offer to join Gowling.

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"An accomplished lawyer with singular insight into the intricacies of policy making, he is uniquely equipped to advise our clients – both domestic and international – on how to successfully navigate Canada's increasingly nuanced regulatory landscape," Gowling Canada chief executive officer Peter Lukasiewicz said.

Under the government of Jean Chrétien, Mr. Reporter served as chief of staff for former minister Allan Rock in the departments of justice, health and industry.