David MacNaughton became ambassador in March 2016.

Canadian Prime Minister Justin Trudeau unexpectedly announced on Thursday that the Canadian ambassador to the United States, who played a pivotal role in the renegotiation of the North American Free Trade Agreement (NAFTA), will leave Washington at the end of August.

David MacNaughton became ambassador in March 2016. Due to his importance in trade negotiations with Canada's closest neighbour and biggest partner, he had cabinet level status in Ottawa. MacNaughton was also an adviser in Trudeau's 2015 campaign.

In a letter, MacNaughton said his decision to leave was not taken "quickly or lightly" and that he "had long planned to complete my work" before Canada's national election on Oct. 21, particularly after the United States lifted its tariffs on Canadian steel and aluminium products earlier this year.

"It is with great affection and gratitude - and of course, considerable regret - that I have accepted Ambassador David MacNaughton's decision to leave Washington, D.C., at summer's end, to return to his home in Toronto, and take up new challenges in the private sector," Trudeau said in a statement.

MacNaughton told reporters at a news conference in Washington that he was looking forward to spending more time with his wife, Leslie, his four grown daughters and six - soon to be seven - grandchildren.

Canada's deputy U.S. ambassador, Kirsten Hillman, will become acting ambassador following MacNaughton's departure, Trudeau said.

Foreign Minister Chrystia Freeland, who worked closely with MacNaughton during the NAFTA talks, praised him for his "steady hand... insight, intelligence, and grit as a negotiator."

"Our country would not have succeeded in the negotiation of the new NAFTA or in securing the removal of the U.S. tariffs on Canadian steel and aluminium without David's leadership, wisdom, and hard work," she said in a statement.

U.S. Trade Representative Robert Lighthizer said on Twitter that "the U.S. has been fortunate to have (MacNaughton) as Canada's representative in D.C."

Canada and the United States have yet to ratify the new North American trade pact, now known as the United States-Mexico-Canada Agreement (USMCA), reached late last year. Canada has said its ratification process will remain in line with the United States.

On Thursday, MacNaughton warned his departure should not be seen as a "premature victory lap" and said the agreement still faces "considerable challenges" such as the need for passage by the U.S. Congress.

A senior government source told Reuters prior to MacNaughton's departure that Canada does not expect movement in Washington on the USMCA for months.

"I won't say that it is a slam dunk by any stretch of the imagination, but I do think that there is broad support even among Democrats," MacNaughton said on Thursday. He said he believes the agreement will pass both U.S. Houses with broad support from both parties.

Canada, he added, will need to ratify the agreement after its federal election.

MacNaughton said Canada was not willing to reopen the North American trade pact, but said officials could consider additional clarifications, like side letters or other "tweaking," within the existing framework.