PA MediaPrime Minister Sir Keir Starmer has said he will be out campaigning for potential Labour leadership rival Andy Burnham ahead of the Makerfield by-election.
Sir Keir said Labour needed to "all pull together and fight" in the by-election taking place on 18 June.
"I want to be part of that, of course I do," the prime minister told reporters on a visit to Essex on Thursday.
A spokesperson for Burnham said: "Anyone who wants to embrace Andy's campaign message is welcome on the campaign."
Sir Keir's leadership was plunged into crisis last week, after dozens of Labour MPs called for him to stand down and Wes Streeting resigned as health secretary, criticising his government's "drift".
The revolt against Sir Keir's leadership came after a dire set of election results that saw Labour lose power in Wales and almost 1,500 council seats in England.
Sir Keir has insisted he will not "walk away" from Downing Street and has sought to seize control of the political agenda with a series of announcements on the cost of living this week.
His intention to canvass voters in Makerfield, in Greater Manchester, is unexpected given Burnham is widely expected to challenge Sir Keir for the Labour leadership, should he become an MP.
When asked whether he would be out supporting the Greater Manchester mayor's campaign in Makerfield, Sir Keir told reporters: "Yes, and I've said to the whole Labour movement that I want everybody to be involved in the campaign, whatever other discussions are going on, it's really important – that's a straight fight between Labour and Reform."
He said Labour has "got a very good story to tell" on the doorstep, referring to recent economic growth figures, falling NHS waiting lists, and a drop in UK net migration to the lowest level since the start of the Covid pandemic.
But in interviews and speeches in recent days, Burnham has been sending a different message.
On Thursday, he told BBC Radio Manchester Britain had been "on the wrong path for 40 years".
In a speech last week, he said "a vote for me will be a vote to change Labour".
Reform UK leader Nigel Farage has already been out campaigning with his party's Makerfield candidate, Robert Kenyon.
Farage has described the by-election as a "David versus Goliath battle" between Burnham and Kenyon, who came second in the constituency in the 2024 general election.
Farage told ITV News Reform UK would "throw the kitchen sink" at the by-election, and said the party's priority in the region would be to ease pressure on social housing.
The Green Party of England and Wales said it was searching for a new candidate after the one it selected, Chris Kennedy, withdrew for personal and family reasons.
The party said it was reopening candidate nominations on Thursday.
A spokesperson said the party would campaign to "expose the risk of Reform" and highlight its policies "to make the super-rich pay their fair share".
A full list of candidates standing in the by-election can be viewed here.
