Sir Keir Starmer has called Boris Johnson “pathetic” for ordering Conservative MPs to abstain from a vote on extending a £20 weekly rise in Universal Credit, and said that “in their heart of hearts” Tories would back Labour’s move.

The Prime Minister was working to lessen the scale of a potential revolt from within his own party as Labour uses an opposition day debate in the Commons on Monday to force a vote calling on the Government not to end the uplift, worth £1,000 a year.

But Conservative former work and pensions secretary Stephen Crabb said he would be voting with Labour, arguing that withdrawing the increase “will cause hardship to families”.

Downing Street insisted no decision has been made on whether to keep or scrap the increase and said Chancellor Rishi Sunak will update the public on the Government’s plans “shortly”.

Mr Johnson was also facing pressure from the 65 Conservative MPs in the Northern Research Group (NRG) who said ending the increase in April as planned would be “devastating”.

Vaccines minister Nadhim Zahawi derided Labour’s symbolic move as a “political stunt”, and the Prime Minister told Conservatives in a WhatsApp message to miss the vote and accused Labour of “playing politics” with “legislatively vacuous opposition debates”.

But the Labour leader told ITV’s Lorraine: “If he’s going to call it a stunt, he (Mr Johnson) should probably come with me to a food distribution centre to see these families this morning and explain to them what is a lifeline to them is a stunt, because it certainly isn’t from their point of view.

“I actually think in their heart of hearts, quite a lot of Tory MPs know that cutting this money to people who desperately need it in the middle of a pandemic is the wrong thing to do, they know that, they probably want to vote with us but because of the tribal way we do politics, they can’t.

“The Prime Minister’s now saying in answer to the question: ‘Do you think this uplift should stay or not?’ he’s saying: ‘I don’t want to say yes and I don’t want to say no, so we’re going to abstain’. He’s got no view on whether it should stay or not – that’s pretty pathetic.”

In a statement on behalf of the NRG, Carlisle MP John Stevenson said the Universal Credit increase had been a “life-saver” for people through the pandemic.

“That is why the NRG are once again calling on the Chancellor to extend the Universal Credit uplift until restrictions are lifted, to ensure that individuals and families who have been worst affected by this pandemic are supported through our recovery with the security they need,” he added.

Mr Crabb told the BBC that when he votes with Labour on Universal Credit, it will be the first time he has rebelled against the party whip.

“I just feel very, very strongly that the current Treasury plan, which is to withdraw that additional amount of money at the end of March, will cause hardship to families, there’s no question about that,” the Conservative MP said.

Senior backbencher Robert Halfon, the Conservative who chairs the Commons Education Committee, said Universal Credit has been “invaluable for millions of people during the lockdown”.

“Depending on what the Government says later, I am most likely to vote for the Labour motion unless the Government makes it absolutely clear it will be extending,” he told the BBC.

The Government temporarily increased the benefit to help families through the Covid crisis, but the uplift is due to expire in April, potentially hitting the incomes of six million families.

The vote would not be binding on the Government, but is being forced by Labour to demonstrate the strength of feeling over the cut in the Commons.

Mr Zahawi told Sky News: “This debate today has no real impact on the outcome on those families, other than a political little stunt for Labour.”

The Prime Minister’s press secretary, Allegra Stratton, also accused Labour of pulling a “political stunt” and insisted no decision has been made on the Universal Credit raise.

“We haven’t made a decision,” she told reporters.

Ms Stratton added: “We know it runs out at the end of March, we know that households want to know what is coming next and he (Mr Sunak) is going to come forward with more information shortly.”

The Resolution Foundation warned that scrapping the £20-a-week uplift will lead to a particularly tough 2021 for low-income households, whose incomes could fall by 4%.

The think tank estimated that the withdrawal of the benefit increase would drive up relative poverty from 21% to 23% by 2024-25, pushing a further 730,000 children into poverty.

Karl Handscomb, senior economist at the Resolution Foundation, said: “The living standards outlook for 2021 looks bleak at present – but the Government can directly improve it.

“Deciding if the £20-a-week uplift to Universal Credit should be extended will determine whether millions of households are able to enjoy any sort of living standards recovery next year.

“And looking further ahead, the decision on whether to keep the UC boost will help define whether this is to be a Parliament of ‘levelling up’ living standards, or pushing up poverty.”