Sir Keir Starmer is facing a leadership coup within hours of Thursday’s by-election in Makerfield if Andy Burnham wins the crunch contest, Labour MPs have said.

Although the Prime Minister has insisted that he will fight off a challenge rather than ‘walk away’, supporters of Mr Burnham are planning a choreographed attack which is designed to make his position untenable.

It comes as the latest poll in the North West constituency shows Mr Burnham five points ahead of his Reform UK rival Robert Kenyon. 

The survey, by More In Common and the UCL Policy Lab, appears to confirm fears within Nigel Farage’s party that they will be denied victory by voters switching to Rupert Lowe’s Restore party. 

The eight per cent saying they will back Restore would make the difference between victory and defeat for Mr Farage.

Under the plans by Mr Burnham’s supporters, more than 100 Labour MPs would come out on Friday morning to call on Sir Keir to set out a timetable for his departure from No 10.

A letter would also be circulated to gather signatures – 81 backbenchers are required to fall behind an individual candidate in order to trigger a leadership contest. 

The MPs hope the letter will be accompanied by further ministerial resignations all the way up to Cabinet level.

Former Deputy Prime Minister Angela Rayner is likely to be jostling with Labour’s Deputy Leader Lucy Powell and former Transport Secretary Louise Haigh for plum jobs in a Burnham Cabinet.

No 10 has been primed for Energy Secretary Ed Miliband and Chancellor Rachel Reeves to potentially follow Defence Secretary John Healey out of the door.

A senior source said: ‘This will go way beyond the usual suspects. The majority of the Parliamentary Labour Party now think the time has come for Keir to go.’

Friday morning could end up being a ‘battle of the press conferences’, with Mr Burnham planning a ‘victory rally’ in Makerfield if he prevails – ostensibly to thank his supporters but in reality an opportunity to provide a springboard for a leadership challenge.

Down in London, the Prime Minister will have to decide whether to make his own public speech, repeating his mantra that he would fight off any leadership challenge.

Sir Keir’s already fragile hopes of clinging on to office were dealt a shattering blow last Thursday with the resignations of Mr Healey and the Armed Forces Minister Al Carns over the UK’s defence investment plan, which Mr Healey said ‘falls well short of what is required for defence and the country at this dangerous time’.

Sir Keir responded by saying: ‘I don’t think we should plunge the country into the chaos of a leadership election,’ adding: ‘I don’t think it should happen, but if it does then I will fight.

‘And let me just be clear... That’s not about personal vanity, it’s not about stubbornness. It’s out of a very deep sense of duty.’

But a Labour source said: ‘Nobody is taking seriously Keir’s statements that he will fight a challenge. 

The reality is that prime ministers always says they will fight on until they don’t. He knows that when the Cabinet pull their support, it’s over. He will accept the inevitable.’

The source added: ‘About two- thirds of the Cabinet want him gone now. And they will start to make their positions known. It will not simply be allies of Andy.

‘There are some people who want the opportunity to launch their own bids, and there are others who want the opportunity to come out in support of other Cabinet colleagues.’