Labour's yawning gap with the Tories in opinion polls is down to last summer's challenge to Jeremy Corbyn's leadership and Theresa May's elevation to Prime Minister, a party source has said.
After Mr Corbyn told colleagues that Thursday's crucial by-elections in Stoke-on-Trent Central and Copeland are on a "knife edge", the source said Labour's poor showing in a succession of polls is "clearly as a result of the internal dissension in the party and the leadership contest".
It comes after an ICM poll for the Guardian showed Mrs May's Conservatives extending their lead over Labour to 18 points, with the Tories on 44% to Labour's 26%.
Mr Corbyn was returned as Labour leader for the second time in two years in September after seeing off a leadership challenge and a string of shadow cabinet resignations after the EU referendum.
After he addressed the weekly meeting of the parliamentary Labour Party (PLP), the source said: "There's obviously concern in the Labour Party at the opinion polls and has been for several months.
"And that's been the case since the leadership challenge last summer, when if you remember, Labour was in the last round of local elections last May, Labour was ahead of the Tories by one point in the actual outcome of the elections.
"And after the leadership challenge and the dissension in the Labour Party, that fell back.
"Obviously since the change of Tory leadership, the Tories have been in a stronger position.
"I think we're confident that won't continue, that Labour's position will improve in the polls and the Tories will fall back.
"I think we're now in a period which won't last, that kind of gap won't last."
Mr Corbyn briefed Labour parliamentarians on plans to expose growing inequality and the "unprecedented" health and social care funding crisis when Chancellor Philip Hammond delivers his Budget next month.
The Labour leader also sought to rally the troops ahead of the by-elections in Stoke, where the party is attempting to see off the threat of Ukip leader Paul Nuttall, and Copeland, where the Tories are looking to win.
"His message was that the by-elections are on a knife edge and that we need to fight for every vote and people need to support Labour's campaign in Stoke and Copeland," the source said.
The source said the Tories' lead in the polls will fall back when Brexit negotiations begin and they are "confronted by the reality" of the rest of the European Union briefing against the Government, which will "exacerbate" internal Tory splits over Brexit.
Meanwhile, a "formal reprimand" letter sent by Labour chief whip Nick Brown to rebellious MPs is intended to be a "last warning" to frontbenchers who defied Mr Corbyn's orders and voted against the Brexit Bill to trigger Article 50, including three junior whips, the source said.