The debate over tax credit cuts is not a constitutional crisis but "a crisis for three million families", Jeremy Corbyn has said.
Mr Corbyn asked the Prime Minister six times to guarantee working families would not be worse off as a result of tax credit cuts as the pair clashed during a heated Prime Minister's Questions on Wednesday.
Mr Cameron said the Government had to make savings to build a strong economy and that the "deficit deniers have taken over the Labour party".
The Government suffered a defeat in the House of Lords on Monday, when a motion to delay £4.4 billion of tax credit cuts while an impact assessment is carried out was passed.
The Prime Minister said Labour had been left to depend on unelected peers and that a new alliance had been created between "the unelected and the unelectable".
He said the Government's new proposals would be set out in November's Autumn Statement.
Mr Corbyn called on the Prime Minister to reveal his plans immediately for the sake of people worried about the cuts.
He said: "This is not a constitutional crisis, this is a crisis for three million families in this country, for three million families who are very worried about what's going to happen next April."
Mr Corbyn pointed to comments made by justice secretary Michael Gove before May's election, when he said the Conservatives would not cut tax credits.
Mr Cameron said the basic level of child tax credits would stay the same.
"If we want to get our deficit down, if we want to secure our economy, if we want to keep on with secure growth, we need to make savings in welfare," the Prime Minister said.
"If you don't save any money on welfare, you end up cutting the NHS, you end up cutting even more deeply policing budgets.
"When is he going to stop his deficit denial, get off the fence and tell us what he'd do?"
Mr Corbyn read a question from full-time worker Karen, who he said accused Mr Cameron of "punishing working families".
"Can he give a cast iron guarantee to Karen and all the other families who are very worried [about] what is going to happen next April to their income, how they're going to make ends meet," said Mr Corbyn. "Please give us an answer to a very straightforward, very simple question."
Mr Cameron said people like Karen would benefit from a range of government measures and the growing economy and that working people would "pay the price" under Labour's plans.