Labour is braced for civil war to erupt today in the aftermath of the most important by-election in decades.

Sir Keir Starmer will defy the growing calls from his leadership rivals and disillusioned MPs to step down, following Andy Burnham’s dramatic attempt to return to Westminster.

His challengers want the embattled Prime Minister to set out a timetable for his departure from No10, to avoid them having to trigger a bitter contest should Mr Burnham triumph in Makerfield, as was expected last night.

Some are hoping that senior ministers will tell him at Tuesday’s Cabinet meeting that his time is up, having become the most unpopular PM in history.

But Sir Keir remains determined to fight on and could try to shut down any discussion of his future around the Cabinet table, as he did last month despite dozens of his MPs telling him to quit.

He and his team insist he alone has the mandate to lead the country, less than two years after he won a landslide general election, and see no reason why he should give it up.

Sir Keir could even issue a direct rebuke to Mr Burnham today if the mayor of Greater Manchester wins in Makerfield and prepares to be sworn in as an MP again on Monday. The PM will not travel to Makerfield – where an estimated 3,000 Labour activists packed the streets yesterday in an unprecedented drive to get voters to the polling stations – as is usual the morning after a by-election victory.

Instead he will speak to broadcasters on a visit elsewhere and, although he is prepared to welcome his rival’s victory, he will say that a leadership contest would be a dangerous mistake.

Labour’s Makerfield by-election candidate Andy Burnham holds rally as a thanks to supporters in Ashton-in-Makerfield 

Sir Keir Starmer (pictured on June 17) will defy the growing calls from his leadership rivals and disillusioned MPs to step down as Labour braces for a civil war

However, if any of his challengers – who also include former health secretary Wes Streeting – decide to trigger one by announcing they have the requisite 81 backers, Sir Keir will take part in it and so try to cling on as Labour leader and PM.

One senior source told the Daily Mail: ‘He will be clear that a challenge would be wrong for the party and the country. But if there was [a contest] he is ready to fight it.

‘I don’t know why Andy Burnham thinks he would have the right to a coronation just because he’d won a by-election.’

Another insider said: ‘He feels a sense of duty and is not going to walk away.’ 

The source said rival camps are desperate for Cabinet ministers to move first and tell Sir Keir he has to go, so they are not seen as disloyal by starting a race.

They also questioned Mr Burnham’s sense of entitlement but predicted he and his team would soon realise the challenge he faces.

‘They’re on a high, thinking they’ve won a by-election and they’re going to take No10,’ the source said. ‘But the reality of coming back to Westminster is going to hit them like a ton of bricks.’

Jockeying is already under way among Labour figures for a place in Mr Burnham’s top team if he takes over. Ed Miliband, who the Daily Mail revealed could quit as Energy Secretary next week to trigger the coup that brings down Sir Keir, is expected to become chancellor.

Louise Haigh, who was forced to resign as transport secretary after it emerged she had a historic conviction for fraud, led the by-election campaign and could be made health secretary.

Anneliese Midgley, once an adviser to Sir Keir, would be chief whip and Mr Burnham’s Manchester policing chief, Kate Green, has been tipped as his chief of staff. 

Last night it was reported that the leadership hopeful – who sparked alarm last year when he said Britain should not be ‘in hock to the bond markets’ – is being advised on the economy by Richard Hughes, former chairman of the Office for Budget Responsibility.

Rachel Reeves, who is keen to remain Chancellor even in a new administration, has warned leadership contenders not to risk changing her fiscal rules.

Former speaker John Bercow, supporting Labour candidate Mr Burnham in Ashton-in-Makerfield

Hugh Grant and Jessica Toale pose with a dog in front of another banner supporting Mr Burnham in the upcoming by-election

Steve Coogan pictured in front of a banner supporting Mr Burnham in the upcoming by-election

She told a conference: ‘We got elected on the promise to return stability to the economy, and anyone who wanted to deviate from that will not be fulfilling the manifesto commitments.’ 

If Mr Burnham does become PM, many expect his honeymoon period to be even shorter than Sir Keir’s. The former MP for Leigh will face calls to hold a general election as he will not be seen as having a mandate, and lack the money or tax-raising powers to implement the Left-wing policies backbenchers want.

Despite Sir Keir’s insistence that he is not going anywhere, some expect he will be forced to step down soon. Resignations could take place in the coming days to pile pressure on him, amid reports that ‘trigger happy’ ministers were told to hold fire over the weekend. Others want senior figures to use Tuesday’s Cabinet meeting to talk the PM into resigning. One source said: ‘People who say privately Keir should go need to start saying that to him, or publicly. The fact is that he doesn’t have the support of most of his Cabinet, his frontbench, the Parliamentary Labour Party or most of the country.

‘The best way forward is to set out a timetable for future-facing candidates to debate ideas, rather than a bitter contest where it’s a blame game for everything that’s happened for the past two years.

‘Keir will go either way, but this way the agency is with him.’