David Cameron has been warned that pensioners' lives could be at risk if cuts were made to the winter fuel allowance.
Speculation surrounds the future of the payments which are being examined as part of a radical shake-up of the welfare system designed to save billions of pounds.
Labour called on the coalition to "come clean" about the plans and public service union Unison's general secretary Dave Prentis warned cutting the winter fuel allowance could cost lives.
Deputy Prime Minister Nick Clegg said savings had to be found in the welfare bill, while stressing that no final decisions had been taken ahead of October's Comprehensive Spending Review.
The coalition agreement between Mr Clegg's Liberal Democrats and the Prime Minister's Tory party pledges to "protect key benefits for older people such as the winter fuel payment", but does not rule out reform.
But Labour pointed out that Mr Cameron had explicitly ruled out scrapping winter fuel payments during the general election campaign.
Shadow foreign secretary and party leadership contender David Miliband said: "Up and down Britain, pensioners rely on this benefit to get by. The payments Labour introduced mean many pensioners can heat their homes without worrying and fretting over the energy bill to come. The Prime Minister's dishonesty is unacceptable."
Mr Prentis said: "We cannot begin to call ourselves a civilised society if the Con-Dem Government allows elderly people to sit in the cold or freeze to death in their homes. They must think again if they are seriously considering cutting these payments, particularly to the very old, because they save lives."
According to reports, the Government is considering raising the age at which people become eligible for the annual winter fuel handout from 60 to at least 66. The payment - worth £250, or £400 for the over-80s last winter - could also be cut by £50 for new recipients and £100 for the oldest.
Interviewed on BBC Radio 4's Today programme, Mr Clegg - covering for Mr Cameron while he is on holiday - said: "It's the middle of August. This is not going to be announced until October. And final decisions will only be taken much nearer the time. I think it actually increases anxieties in having this constant running commentary on decisions that haven't been taken or might not be taken at all."