Andy Burnham plans to give Sir Keir Starmer one final chance to 'do the decent thing' and resign before orchestrating a putsch to remove him from Downing Street.

The Daily Mail can reveal that he is planning to deliver the PM a private ultimatum this weekend, warning him that he must either set out a timetable for departure or face an imminent coup.

The Greater Manchester mayor will stop short of publicly calling for the Prime Minister to resign if he wins Thursday's Makerfield by-election.

But his supporters will be despatched to the airwaves to pile pressure on the beleaguered PM. Allies are then plotting a wave of ministerial resignations to try to force him out.

Leadership rival Wes Streeting also called for the PM to set out a timetable this week. In a speech in London, the former health secretary said it was time to end the 'drift and uncertainty' at the top of the Labour government. 'When the results [from Makerfield] are in, I hope the Prime Minister will, at that stage, reflect on his own position and set out a timetable,' he said.

'I think that would be a better way forward for everyone.'

However, Sir Keir insisted again on Tuesday that he will fight any leadership challenge, potentially setting the scene for a struggle for power in which the needs of the country could be relegated for months.

More than 100 Labour MPs have called for him to quit following last month's disastrous local election results. But speaking to reporters at the G7 summit in France on Tuesday, the PM said: 'I've been very clear throughout this that we won a significant General Election result in 2024 with a mandate to bring about change. I'm not going to walk away from that, so I will fight if there's a challenge. I don't think there should be a challenge.'

Allies of Mr Burnham claim he is determined to be in Downing Street by the time of the Labour Party conference in September at the latest. He is pushing for a bloodless 'coronation', arguing it will prevent Labour tearing itself apart.

Reports have claimed that Mr Burnham will launch a leadership campaign 'within hours' if he sees off a stiff challenge from Reform in Makerfield.

But Labour sources told the Daily Mail that he will start by asking supportive MPs to pile pressure on Sir Keir by calling for him to quit as soon as the Makerfield result comes through in the early hours of Friday morning.

He is then pressing for private talks with Sir Keir at the weekend where he will urge him to resign 'with dignity'.

However, relations between the two men are in the deep freeze.

Sir Keir is said to be furious at Mr Burnham's decision to publicly set out his ambition for power during the by-election. Allies claim the 'betrayal' led to him ditching a promise to campaign for Mr Burnham in the constituency.

If the PM refuses to budge, then Mr Burnham's supporters could trigger a wave of ministerial resignations starting next week to try to force him out. Only if Sir Keir refuses to go will Mr Burnham seek the 81 nominations needed to mount a formal leadership challenge under Labour's rules.

Mr Streeting, who quit the Cabinet last month, insisted he also has the support needed to trigger a leadership contest. And he

indicated he will try to ensure a contest even if Sir Keir does a deal with Mr Burnham to stand down. Mr Streeting acknowledged that a Labour contest could descend into a damaging slanging match that results in a 'Dutch auction' of unaffordable promises.

But, asked if he could step aside to allow Mr Burnham a coronation, he told the Daily Mail: 'I think we are better off if our ideas are tested and there is that kind of genuine contest. I think the Labour Party will be better served... it doesn't have to be a bitter contest.'

He added: 'I think there should be a contest. I have every intention of standing in that contest.'

Meanwhile, a major union joined Angela Rayner in pushing for Labour to veer to the Left.

Unison chief Andrea Egan – a self-proclaimed 'fan' of Mr Burnham – linked funding for the party to a 'course change' for more 'progressive' policies. She suggested that included more public sector pay hikes and a U-turn on tough immigration curbs.

Meanwhile, in a speech to the union's annual conference in Brighton, Ms Rayner insisted her workers' rights overhaul was 'just the start'.

She said the Government had to be 'active and interventionist' to increase wages by tackling 'the vested interests ripping us off'.