Keir Starmer was last night warned he faces a coup within days unless he steps aside for Andy Burnham.

Labour grandees, including some Cabinet ministers, told the Prime Minister that his time was up following Mr Burnham's thumping victory in the Makerfield by-election.

Transport Secretary Heidi Alexander is among those understood to have told Sir Keir yesterday that he needed to set out a timetable to quit this summer.

A senior Labour source told the Daily Mail that the PM faced a showdown at Tuesday's Cabinet meeting unless he had agreed to step down by then. 'The challenge for the next few days is to force him out and get him to accept the reality of his position,' the source added. 

'If he is still digging in his heels then you will see things come to a head at Cabinet. People are not going to be fobbed off this time, because the choice is either a fairly orderly transition or an extremely bitter contest that damages the entire Government.'

One Cabinet source said, 'the game is up', while a minister described Sir Keir as a 'dead man walking' and said he was ready to quit if the PM tried to cling on.

Mr Burnham vowed to set 'a new path for Britain' after comfortably holding off a challenge from Reform UK to return to Westminster.

The self-styled 'King of the North' said his election as MP for Makerfield was the 'final chance' for Labour to reconnect with the public after a disastrous first two years in power, with his victory set to be a 'turning point' for the country.

His anointment as Labour leader is expected to result in a further lurch to the Left. He has already committed to a string of costly policies such as nationalisation of the utilities, massive investment in council housing and revival of the northern leg of HS2 – raising fears he will push taxes even higher.

His right-hand woman, former transport secretary Louise Haigh, warned that the PM faced a 'brutal' end unless he agreed to step down. She said Mr Burnham had a leadership campaign 'ready to go', although his supporters are privately pushing for him to be crowned in a coronation to avoid a bloody contest.

But Sir Keir vowed to fight on, saying a leadership contest would plunge the party into 'chaos'. The PM said he had a 'very strong mandate' from the country, adding: 'If there is a contest, I will stand. I have said repeatedly, I am not going to walk away from that.'

Sir Keir will spend the weekend at Chequers weighing up his future – and wargaming how to deal with a potential wave of ministerial resignations next week.

Former frontbencher Andy McDonald said the PM 'has got days' to quit with dignity before he was forced out. And Harriet Harman, recently brought back into government by Sir Keir, warned that Labour MPs were 'stampeding' towards his rival.

On a day of political drama:

• The number of Labour MPs calling for the PM to resign topped 100, with even former loyalists joining in;

• Mr Burnham put nationalisation at the top of his agenda, as he set out a mini manifesto that also included the 'reindustrialisation' of the North;

• Kemi Badenoch was celebrating a historic victory in the Aberdeen South by-election, which had been dominated by controversy over Labour's opposition to new North Sea oil drilling;

• Nigel Farage said he was 'disappointed' with Reform's showing in Makerfield, which had been a key target seat;

• Mr Burnham was weighing up whether to install Ed Miliband as chancellor or pick a less radical figure such as work and pensions secretary Pat McFadden or fellow minister Darren Jones; Baroness Harman called for Labour members to be locked out of any leadership contest, saying a decision on the next Prime Minister should be left to MPs.

• Allies of Wes Streeting said he was still set on forcing a contest to ensure any new leader had a mandate for their ideas for the country;

• Members of Labour's hard-Left rallied behind Mr Burnham in the expectation that he will ditch the more moderate parts of the party's manifesto and turn on the spending taps.

Former Labour health secretary Alan Johnson told Mr Burnham he should call an election this year to 'seek his own mandate' if he wanted to deliver real change. He told LBC that, after six prime ministers in a decade, the public 'will not stand' for having a seventh imposed on them.

Makerfield had been billed as a tight contest, but Mr Burnham won comfortably with 55 per cent of the vote.