David Cameron has said he has no plans to cut child benefit and tax credits if the Conservatives win the election.
It comes after the Lib Dems leaked a document which they claimed showed the Tories wanted to make £8bn of cuts to child benefit three years ago.
Mr Cameron said he rejected the idea back then "and I reject it again today", in a BBC TV election special.
But he said he wanted to continue reforming welfare, insisting there was room to make more savings.
Labour said the prime minister had still not ruled out cuts to child tax credit.
The last of the set piece leaders' TV appearances came after Lib Dem Chief Secretary to the Treasury Danny Alexander claimed he was "lifting the lid" on Tory plans to limit child benefit and tax credits to two children.
Mr Alexander, who is effectively Conservative Chancellor George Osborne's deputy in the Treasury, claimed his party blocked the move, which he said was worth £8bn.
The prime minister was grilled on his party's welfare plans during the BBC Question Time leaders special, which also featured Labour leader Ed Miliband and Lib Dem leader Nick Clegg in separate appearances.
Mr Cameron said he wanted to end the speculation on cutting child benefit: "I don't want to do that. This report that was out today is something I rejected at the time as prime minister and I reject again today."
He said child tax credit had increased under his government by £450, telling the audience: "That's not going to fall."
And he added: "Child benefit to me is one of the most important benefits there is. It goes directly to the family, normally to the mother, £20 for the first child, £14 for the second.
"It is the key part of families' budgets in this country. That's not what we need to change."
'Wrong thing to do'
However, the PM did make the case for further reforms to welfare, saying that when he took office some families were receiving up to £90,000 in housing benefit - which he said was not acceptable.
The Conservatives say they would cut £12bn from the welfare budget if they remain in power to help plug the deficit, but they have faced criticism for not explicitly setting out where the cuts will come from.
Taking questions on stage, Mr Cameron insisted: "We can reduce welfare if, for instance, we get another two million back to work. That will cut welfare bills."
He warned that failure to make cuts to welfare would mean deeper cuts elsewhere, to areas like the NHS.
"If you believe as I do that we should go on reforming welfare, making sure that work always pays, helping people back into work and keeping working people's taxes down, that's my programme.
"If you want unlimited welfare, more increases in welfare and higher taxes for working people that's Ed Miliband's programme, vote for him," he said.
Appearing on the same programme later, Labour leader Ed Miliband said the prime minister had not been able to guarantee that he would not cut child benefit or tax credits.
But he pledged: "I'm not going to cut your tax credits, I'm not going to cut child benefit or means-test it. I think that would be the wrong thing to do for our country, the wrong thing to do for family finances."
Earlier in the day, Chancellor George Osborne rejected the Lib Dem claims, insisting that said there would be "no cuts to child benefit" if the party remained in power.
"What we instead have proposed is that working age benefits are frozen for the next two years. We think that's a more equitable, fairer way of making sure we contain the costs of our welfare system," he said.
Speaking to BBC London Mr Osborne said the document Mr Alexander was using to substantiate his accusations was "commissioned by the chief secretary himself" - however a Lib Dem source speaking to the BBC disputes this, and claims that it was in fact commissioned by the prime minister.
The chancellor added: "We haven't put into practice any of these options, we don't support them. We didn't support them then and we don't support them in the future.
"But there is a clear choice in this election. Ed Miliband wants to put up the benefits bill and that means higher taxes for working people. David Cameron and the Conservatives will cut taxes for working people and we think savings can be made in the welfare system."
The Tories say their target of cutting £12bn from the welfare bill was achievable, because £21bn had been saved over the past five years.
But they have yet to specify where the savings will be made, and Mr Alexander said the list of measures he revealed to the Guardian "shines a light" on the scale of cuts the Tories would have to make to working-age benefits.
The Guardian said the measures were contained in a paper circulated to the four most senior cabinet members by Work and Pensions Secretary Iain Duncan Smith in 2012.
Mr Alexander said the proposals also included removing child benefit from 16 to 19 year-olds, as well as means testing the payment.
The coalition government has already restricted child benefit - aimed at helping parents cope with the cost of bringing up children - for parents earning £50,000 a year or more.
By Michael Buchanan, BBC social affairs correspondent
Officials at the Department for Work and Pensions have struggled for months with how to cut £12bn from welfare spending should the Tories win the election. One option they'll present to any incoming Conservative ministers is to scrap child benefit for all but the poorest families. The change would see only those people receiving Universal Credit getting child benefit, protecting the poorest third of families.
But two thirds of families who currently get the benefit - households whose earnings were about £40,000 or more - would completely lose the benefit. Officials calculate the change would save about £4bn per annum. The difficulties of saving £12bn have been exacerbated by low inflation. The major detail given by the Tories - to freeze working age benefit levels for 2 years - was predicted to save £3bn when announced by the Chancellor last September. However a recent calculation by the IFS has found that low inflation has reduced that saving to nearer £1bn.
Reality check:How easy is it to find £12bn of cuts?
Mr Alexander said: "I'm lifting the lid on this now because the Conservatives are trying to con the British people by keeping their planned cuts secret until after the election."
He told BBC Radio 4's Today programme that the Tories were either being "reckless or deceitful" in not divulging the details of how they would find £12bn of savings from the welfare budget.
"These are ideas that were put to me by Conservative ministers," Mr Alexander said, but admitted "we probably did know some of this before".
Paul Johnson, the director of the Institute of Financial Studies (IFS), has previously said that only £2bn of the proposed £12bn cuts had been outlined by the Conservatives so far.
In response to Mr Alexander's allegations, SNP leader Nicola Sturgeon said: "Danny Alexander has got a bit of a cheek. He's been part and parcel of the government for the last five years that has introduced and imposed these savage cuts.
"I think people want change certainly from the Tories and I think they want a change from the Lib Dems as well."
* Subscribe to the BBC Election 2015 newsletter to get a round-up of the day's campaign news sent to your inbox every weekday afternoon.