Real-terms welfare cuts are "fair and right", David Cameron insisted as he faced Labour accusations the Government was trying to "divide the country".
MPs voted on Tuesday night to cap rises in most working-age benefits at a below-inflation 1% for three years - ending the usual link to the cost of living. Charities which work with poor families voiced dismay at the outcome and there was a small-scale rebellion by some Liberal Democrat MPs.
In clashes at Prime Minister's question time, Opposition leader Ed Miliband attacked the changes, which his party voted against.
He said: "The Chancellor hits hard-working people and the most vulnerable with his 'strivers' tax' but at the same time he is giving a massive tax cut to millionaires. You have broken that most symbolic promise that we are 'all in this together'".
But Mr Cameron said it was "inexplicable" that Labour opposed the squeeze but had supported a 1% pay rise for public sector workers. He said: "Over the last five years, benefits have gone up by 20% yet average earnings are only up by 10%. So I think it is fair and right to have a 1% cap on out-of-work benefits, a 1% cap on tax credits and, of course, the 1% cap on public sector pay.
"What I think is inexplicable is the position of the party opposite to support a 1% public sector pay cap but to want more for welfare claimants. That is not fair, it is not right, and they should think again."
The cap, announced by Chancellor George Osborne in his Autumn Statement last year, is aimed at slashing £5 billion from the welfare bill over the next five years. Mr Osborne has previously justified it by appealing to workers annoyed at seeing their neighbours blinds closed and "sleeping off a life on benefits" when they go to work.
Critics - including some on the Tory benches - accuse him of portraying the jobless as "shirkers" in a bid to gain public support for the measure.
The row over the impact of the cap on working families has heightened pressure on Mr Cameron to commit to scrap universal benefits for pensioners. Tory backbenchers are among those declaring it "mad" that well-off OAPs continue to enjoy free bus passes, winter fuel payments and television licences at a time of austerity.
Deputy Prime Minister Nick Clegg has voiced similar concerns and Work and Pensions Secretary Iain Duncan Smith appeared on Tuesday to hint reforms could be in the 2015 Tory manifesto. Mr Cameron has repeatedly insisted he will not abandon a 2010 pre-election pledge to protect such perks for the duration of this parliament.