Keir Starmer has branded Boris Johnson "pathetic" and told him to explain in person to affected families why he wants to cut their support payments by over £1,000 a year.
Speaking ahead of a vote on whether to reduce Universal Credit payments for six million families the Labour leader said the prime minister should come with him and speak to those struggling face-to-face.
Mr Johnson had accused Labour of a "stunt" for forcing a vote on the £20-a-week cut later today.
"If the prime minister is going to call it a ‘stunt’ he should probably come with me to the food distribution centre this morning and explain to me why he thinks that what's a lifeline for them is a 'stunt'. It certainly isn’t from their point of view" Sir Keir told ITV's Lorraine programme.
Sir Keir said the £20 a week uplift, introduced last year but now set to expire, was "the difference between being able to pay the gas, electricity, and internet bill combined" for many families.
The Labour leader said he suspected that Tory MPs, who are being whipped to abstain on the Labour motion, would prefer to vote with the opposition but were being blocked by the government.
"Here's the truth: 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," he told the programme.
"They know that, they probably want to vote with us but because of the tribal way we do politics they can't.
"So the prime minister is now saying in answer to the question 'do 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. No view on whether it should stay in place or not.'
"I think that's pretty pathetic and a lot of Tory MPs are worried about this. They know as constituency MPs they all do a surgery ... they know that families desperately need this money."
Some Conservatives have criticised the government and urged Mr Johnson and his chancellor Rishi Sunak to change course.
Tory MP and former Work and Pensions Secretary Stephen Crabb told Times Radio on Monday morning now was "not the moment to withdraw that money from these families".
A report by the Resolution Foundation think-tank on Monday warned that the government would be pushing Britain’s poorest households further into poverty if the cut goes ahead.
A further 1.2 million people, including 400,000 children, will fall into relative poverty, while levels of absolute poverty will rise in a way they did not even in the hard times of the 1980s, according to the think tank’s analysis.
Removing the UC uplift will return the basic level of unemployment benefit to its lowest real-terms value since the early 1990s. And it will mean incomes among the poorest 10 per cent have not increased at all since the early 2000s, after inflation is taken into account.
Withdrawal of the UC uplift will drive up relative poverty from its current 21 per cent to 23 per cent by the time of the election, forcing a further 730,000 children below the breadline and bringing relative poverty to its highest level since 2000, the report found.
The motion put forward by Labour is not binding but it would be politically difficult for the government to ignore a defeat in parliament.