A sitting Liberal MP has written to the federal caucus to say he thinks Prime Minister Justin Trudeau should resign.

"For the future of our party and for the good of our country, we need new leadership and a new direction," said New Brunswick MP Wayne Long in the brief note obtained by CTV News.

"The voters have spoken loud and clear, they want change. I agree," he said.

Long has already said he doesn't plan to run again. He joins former Trudeau Liberal cabinet minister Catherine McKenna and a small but growing list of other party faithful who have come out on record in the days since the shocking Toronto-St. Paul's byelection defeat, to call on the prime minister to step down.

"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," McKenna said in a statement to CTV News. "The prime minister has a legacy to be proud of but it's time for new ideas, new energy and a new leader. There is too much at stake in this election especially on the economy and climate."

McKenna has recently been quoted saying that former Bank of Canada governor and much-rumoured leadership contender Mark Carney would make a good candidate.

Long said he wrote to his fellow MPs so they knew "clearly and directly," where he stood.

Other MPs have quietly expressed serious concern about the party's political and electoral fortunes going forward – either without some sizeable structural shakeups to his front bench and inner circle, or under Trudeau at all – while cabinet ministers have been lining up behind him, when asked.

Earlier this week, without addressing the leadership questions head-on, Trudeau said he's heard people's concerns and acknowledged that he and his team "have much more work to do to deliver tangible, real progress that Canadians across the country can see and feel."

The Prime Minister's Office has reportedly been doing outreach to parliamentarians this week, in the absence of a caucus-wide meeting.