Sir Keir Starmer has told supporters this weekend he will stand in any Labour leadership contest that is triggered.
Until now, the prime minster and his allies insisted he would not walk away if one was held but there has been a big question mark over what he would actually do.
However, after Greater Manchester Mayor Andy Burnham said he would seek to enter any potential leadership contest, Sir Keir's position hardened, the BBC understands.
The prime minister, who still believes a leadership contest could plunge the country into chaos, said he had won a massive majority two years ago and a mandate that he was determined to deliver.
With former Health Secretary Wes Streeting and Burnham circling, and scores of Labour MPs having urged the prime minister to quit, there have been weeks of speculation about a possible challenge.
Much of what happens next depends on the outcome of the Makerfield by-election on 18 June, which Burnham must win to be able to stand in a leadership contest.
But the prime minister's resolve to fight means the chances of a full-blown Labour leadership scrap seem to have risen.
As the sitting Labour leader he would automatically be on the ballot that goes to Labour party members.
There has not yet been a leadership challenge but speaking on the the BBC's Any Questions programme on Thursday, Burnham indicated strongly that he would stand for leader.
He said: "I think Wes Streeting seems to have launched a leadership contest, so if that is running, I would seek to join it. But I'd have to persuade members of the Parliamentary Labour Party to do the same."
Streeting has confirmed he would enter any potential contest and has encouraged people to back Burnham in Makerfield as he wants a "proper contest with the best candidates on the field".
Pressure has been mounting on the prime minister after a historically bad set of election results last month, in which Labour lost control of the Welsh Senedd, as well as nearly 1,500 councillors in England.
Streeting and a handful of junior ministers quit the government in the aftermath, while Burnham, regarded as the frontrunner in any future leadership contest, set his sights on returning to Westminster in the Makerfield by-election.
-BBC
Until now, the prime minster and his allies insisted he would not walk away if one was held but there has been a big question mark over what he would actually do.
However, after Greater Manchester Mayor Andy Burnham said he would seek to enter any potential leadership contest, Sir Keir's position hardened, the BBC understands.
The prime minister, who still believes a leadership contest could plunge the country into chaos, said he had won a massive majority two years ago and a mandate that he was determined to deliver.
With former Health Secretary Wes Streeting and Burnham circling, and scores of Labour MPs having urged the prime minister to quit, there have been weeks of speculation about a possible challenge.
Much of what happens next depends on the outcome of the Makerfield by-election on 18 June, which Burnham must win to be able to stand in a leadership contest.
But the prime minister's resolve to fight means the chances of a full-blown Labour leadership scrap seem to have risen.
As the sitting Labour leader he would automatically be on the ballot that goes to Labour party members.
There has not yet been a leadership challenge but speaking on the the BBC's Any Questions programme on Thursday, Burnham indicated strongly that he would stand for leader.
He said: "I think Wes Streeting seems to have launched a leadership contest, so if that is running, I would seek to join it. But I'd have to persuade members of the Parliamentary Labour Party to do the same."
Streeting has confirmed he would enter any potential contest and has encouraged people to back Burnham in Makerfield as he wants a "proper contest with the best candidates on the field".
Pressure has been mounting on the prime minister after a historically bad set of election results last month, in which Labour lost control of the Welsh Senedd, as well as nearly 1,500 councillors in England.
Streeting and a handful of junior ministers quit the government in the aftermath, while Burnham, regarded as the frontrunner in any future leadership contest, set his sights on returning to Westminster in the Makerfield by-election.
-BBC
