David Cameron has sidestepped questions over whether tax credit claimants will still be worse off when measures to soften the impact of reforms are announced next month.
The Prime Minister repeatedly refused to say if the some of the country's poorest households would still lose out under the reforms aimed at cutting £4.4 billion off the welfare bill.
Labour leader Jeremy Corbyn told the premier "people are very worried about what's going to happen to them" and called on him to make a cast-iron guarantee that claimants would not be hit financially.
Mr Cameron said: "We will set out our new proposals in the Autumn Statement."
Mr Corbyn accused the Prime Minister of failing to answer his question six times and called for a "cast iron guarantee" for the families set to lose out from the cuts.
The Labour leader said Mr Cameron had "lost the support of many people in this country who are actually quite sympathetic to his political project" over the tax credits cuts.
The Prime Minister claimed that MPs had voted for the measures "five times" with "absolutely no rebellion" from Tories, leaving Labour to depend on "unelected peers in the House of Lords".
In a jibe at Mr Corbyn he said: "In British politics we have got a new alliance: the unelected and the unelectable."
But the Labour leader hit back: "This is not a constitutional crisis, this is a crisis for three million families in this country."
The Prime Minister accused the Labour leader of having a "deficit-denying, borrow forever plan".
The Labour leader asked a question submitted by public sector worker "Karen" who claimed the PM was "punishing working families".
Mr Corbyn called for Mr Cameron to give a "cast iron guarantee to Karen and all the other families who are very worried what is going to happen next April to their income."
Mr Cameron told him: "The reason the Labour Party lost the last election is because they are completely untrusted on the deficit, on debt and on a stable economy. Since then the deficit deniers have taken over the Labour Party, that is what happened.
"When you look at their plans: borrowing forever, printing money, hiking up taxes - it is working people like Karen that would pay the price."
Downing Street insisted that Mr Cameron "did answer" Mr Corbyn's questions but Labour said the PM had failed to give a "clear" response.
A No 10 spokesman said there were no plans to re-open areas of ring-fenced funding, such as schools and hospitals, to make up any shortfall created by the introduction of transitional measures to help claimants.
But he added there are "no easy answers" and insisted "we can't turn our backs and pretend the deficit will just go away".
Chancellor George Osborne was given a rousing reception when he attended a meeting of the Conservative backbench 1922 Committee, with MPs banging their desks in a traditional show of support.
One MP present said that the mood had been "100% supportive" for the Chancellor in the wake of the the Government's defeat in the Lords.