Allies of Angela Rayner expect Sir Keir Starmer to be ousted within 12 months – with the Deputy Prime Minister most likely to succeed him in No 10.
Last week's humiliating Government U-turn over planned £5 billion cuts to welfare has left the Prime Minister 'gravely wounded' and unlikely to lead the party into the next election, the allies said.
After rebel MPs forced Sir Keir to row back on the reforms, Chancellor Rachel Reeves has been left with a further £3 billion black hole in her finances, raising the prospect of more tax rises in the autumn.
Despite his concessions, party sources say the PM is still not guaranteed to win a Commons vote on the reforms on Tuesday.
The turmoil – ahead of Sir Keir's first anniversary as Prime Minister on Saturday – comes as a poll for today's Mail on Sunday found that 61 per cent of voters think Sir Keir should quit as Prime Minister.
According to the survey by Find Out Now, only 25 per cent think Sir Keir will still be in Downing Street by the next election, with even Labour supporters split 50/50 on the question. Futhermore, a total of 64 per cent think that Ms Reeves should be fired by Sir Keir.
Ms Rayner tops the list of ministerial candidates to succeed Sir Keir among all voters – and boasts commanding support from the party and union members who would vote in a leadership contest.
The Rayner ally told The Mail on Sunday: 'I think Angela will be the leader.
After next year's Welsh and local elections, if Starmer does not stand down, he will face a challenge. People have just had enough and Keir is gravely wounded.'
The MP added that while the Left would prefer Manchester mayor Andy Burnham to succeed Sir Keir, he is not currently an MP, making Ms Rayner 'the most acceptable and most viable option'.
They rejected Ms Rayner's insistence that she had 'no desire', saying: 'That's what she would say' – adding that of all the potential Labour leaders, she could challenge Nigel Farage as 'a genuine working-class woman against a man who just wants working-class votes'.
But one MP close to Ms Rayner insisted last night that she was focusing on her government job, and dismissed talk of leadership manoeuvreings as nonsense.
Further complicating the picture was a poll last week which suggested Ms Rayner could lose her Ashton-under-Lyne seat to Mr Farage's Reform party.
Labour MPs claim that the concessions to exempt existing Personal Independence Payment recipients from the cutbacks would not necessarily ensure the measures will pass on Tuesday.
Labour rebels were still threatening to table a new amendment to kill off what was left of the legislation, with suggestions that as many as 50 Labour MPs would back the move.
But party insiders insist the Government will now narrowly win the vote.
Sir Keir is under pressure from both the Left and Right of his party.
Ms Rayner's nearest rival, Blairite Health Secretary Wes Streeting is said to have increased his personal contact with Labour MPs.
A source said: 'It is always about health reforms, admittedly, but we suspect a hidden motive'.
One MP said last night that the Prime Minister's authority over his party was now 'shot to pieces'.
Another contrasted Ms Rayner with the PM, saying that while 'Angie's a politician', Sir Keir relied 'on other people' to do his political thinking for him.
And a third backbench MP accused Sir Keir of being 'more interested in his own agenda' of assisted dying legislation than the Government's official programme.
'He was so tuned out of the welfare stuff that the rebellion was all a surprise,' the MP added.
Veteran Left-wing MP Diane Abbott called on the PM to 'look at Tony Blair's leadership. [He] knew he had to listen to his MPs... Starmer doesn't appear to understand that until it's really very late'.
Former Labour Cabinet Minister Lord Falconer said that after last week's events, 'there needs to be change' to show the Government is working with its MPs.
Sir Keir's nightmare continued yesterday when he was barracked by pro-farming, fishing and Palestinian protesters outside the Welsh Labour conference.
