Angela Rayner is telling Labour MPs she is ready to launch a leadership campaign, despite remaining under investigation over her tax affairs.

In recent days, the former deputy prime minister has signalled she could fall in behind Greater Manchester Mayor Andy Burnham in any leadership contest.

But Labour sources have told the Mail that she is still canvassing MPs for support for her own campaign if Keir Starmer is forced to quit at short notice.

‘Angie was struggling for support, but she will be a big beneficiary if things move quickly,’ said one. ‘Publicly, she is making very loud representations that Andy should be allowed back. But she is also texting everyone saying she is ready to go.

‘If Andy does get back, she would probably have to row in behind him. But there is a scenario where he can’t get back in time and then, as she sees it, the hand of history falls on her shoulder.’

Ms Rayner was forced to quit the Cabinet last year after it emerged she had failed to pay £40,000 in stamp duty owed on a luxury apartment in Hove, 260 miles from her Greater Manchester constituency.

The case is currently under investigation by HM Revenue and Customs. But allies believe she will eventually be cleared of wrongdoing – and insist that the drawn-out investigation will not prevent her from launching a bid to become prime minister.

Supporters believe her popularity with Labour members would put her in a strong position to beat Wes Streeting in a leadership contest.

However, polling suggests that the tax row has dented Ms Rayner’s popularity with the wider electorate. A YouGov poll last week found the public now have an unfavourable view of her by a margin of 56:21 - worse than either Mr Burnham or Mr Streeting and only slightly better than Sir Keir.

One ally of Mr Streeting questioned the wisdom of installing Ms Rayner to succeed Sir Keir, whose downfall has been accelerated by controversy over his disastrous decision to appoint Peter Mandelson as US ambassador.

‘Do people really want to replace one scandal-hit leader with another?’ the source said.

Rival camps also say Ms Rayner’s chances have been dented by revelations in the Mail this month that she stumbled heavily into a door after a long night of socialising in parliament’s Strangers Bar.

One MP said: ‘That incident has reopened all the old questions about whether she really has the temperament for the top job.’

Ms Rayner is said to have signed a non-aggression pact with Mr Burnham, pledging that she will not stand against him. Sources suggest that, in return, he has promised to return her to the role of deputy prime minister if he seizes power.

In a speech on Monday, she savaged the Prime Minister for blocking Mr Burnham’s bid to return to Westminster at the Gorton and Denton by-election this year – and suggested he should now be allowed to return.

Ms Rayner said the decision to bar Mr Burnham in February was a ‘mistake that the leadership of our party should put right’.

On Sunday, she issued a mini-manifesto in which she said the PM had to ‘meet the moment’ with bolder action to tackle inequality and ‘squeezed living standards’, which she insisted could be done ‘within the current fiscal rules’.

In her 1,000 word statement, she called for Labour to raise the minimum wage, offer regional mayors more economic powers and be ‘unafraid to promote new forms of public, community and cooperative ownership across the board’.