The prime minister has defended the decision to compensate people who have lost money from tax changes, saying he was not "pushed about" by Labour MPs.
Gordon Brown told the BBC it was important to "judge what is right at every point".
Earlier, facing a possible Labour rebellion over the abolition of the 10p income tax rate, the government promised more help to low earners.
The Conservatives accused Mr Brown of "weakness, dithering and indecision".
In what has been widely seen as a U-turn forced by Labour rebels, Chancellor Alistair Darling said he would assess the loss to pensioners aged 60 to 64 and childless people from the abolition of the 10p rate.
The compensation deal will be unveiled in the autumn and backdated to April.
Do you have any idea what a pathetic figure you cut today?
"You are making these changes because you thought you would lose the vote."
Quoting a number of Labour figures who have questioned the prime minister's handling of the affair, Mr Cameron said: "Isn't it the case that the Labour Party have finally worked out that they have a loser, not a leader?"
Mr Brown countered that Mr Cameron's "new found" interest in poverty had "lasted a few seconds".
He said the 10p rate was not the best way to boost the incomes of the low paid, who had benefited from improvements in tax credits and the minimum wage under Labour.
'Penalising the poor'?
"The central issue is we have taken more people out of poverty than any previous government," he told the Commons.
"The choice is very clear - between a Conservative Party that would cut the incomes of the poor and a Labour Party that will increase them."
In a further attack, Liberal Democrat leader Nick Clegg accused Mr Brown of "doing the Tories' job for them" by "penalising the poor".
Ex-minister Frank Field explains why he decided to withdraw his amendment
"You used to be a man of principle, but if you won't deliver on poverty, what on earth is the point of this increasingly pointless prime minister?"
"The point is to have economic growth in this country, that gets more people into work and that could not happen under Liberal policies," replied Mr Brown.
The government could offer a higher winter fuel allowance for pensioners aged 60 to 64 and new tax credits for low-paid workers without children.
But, while the Treasury has confirmed that it will back date extra winter fuel allowance payments, it said it is not in a position to say it can do the same for the tax credits.
Some Labour MPs have expressed concern because they were under the impression that all the compensation would be backdated.
Younger workers have been offered the prospect of a higher minimum wage paid for by employers - the Conservatives say this would affect only 172,000 workers, a small fraction of those affected.
In an interview with the BBC later, Mr Cameron said he would "welcome anything" that would help those worst hit.
But he said: "We will be checking to make sure there still aren't losers from the Budget and from what he's planned, because with this prime minister always check the small print."
In a letter outlining his proposals, the chancellor said the introduction of a "more generous" tax credits system had meant the 10p tax rate, as a means of helping low-income households, had outlived its usefulness.
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