A backbench MP has become the first sitting member of Canadian Prime Minister Justin Trudeau’s Liberal Party caucus to call on him to resign, after a disastrous by-election defeat.

Wayne Long said new leadership was needed “for the future of our party and for the good of our country” in an email to fellow party members that was obtained by Canadian media outlets.

Mr Long, who represents a riding in the province of New Brunswick, wrote “the voters have spoken loud and clear and they want a change”.

It follows a catastrophic result for Liberals in a by-election on Monday, when they lost a Toronto seat they had held since 1993.

Mr Long has served as MP since 2015 and previously said he would not be seeking re-election.

According to the National Post, Newfoundland MP Ken McDonald replied all to Mr Long's email, saying: "well said!"

In recent days former Liberal politicians have also called on Mr Trudeau to step down.

Among them was Catherine McKenna, who served as environment minister under Mr Trudeau from 2015-19.

“The Liberal Party isn't about one person. It's about the values it stands for and it's about improving the lives of Canadians,” said Ms McKenna in a statement she shared with media outlets.

“The prime minister has a legacy to be proud of, but it's time for new ideas, new energy and a new leader.”

Former British Columbia Liberal Premier Christy Clark made a similar statement on Thursday in an interview with the Toronto Star.

Ms Clark said the prime minister “does not have the pull with voters he once had, but more importantly, he isn’t even able to motivate some of the people in the country who are most likely to vote for his party”.

Canada’s next election must take place by 20 October 2025.

After Monday’s stunning defeat, Mr Trudeau - who has been prime minister since 2015 - vowed to stay on as Liberal leader.

“These are not easy times,” he said. “And it is clear, I and my entire team have much more hard work to do to deliver tangible, real progress that Canadians can see and feel."

The prime minister has not taken questions from reporters since the loss.

The Conservatives snatched the Toronto-St Paul's seat by a little more than 500 votes.

Mr Trudeau has seen a significant drop in his popularity because of a host of issues including cost-of-living pressures.

A recent Ipsos poll for Global News suggested that as many as 68% of Canadians want Mr Trudeau to step down.