Chancellor George Osborne is seeking cuts totalling £10 billion for the financial year 2015/16, and is currently locked in negotiations with Cabinet colleagues over where the axe will fall.
New doubts have been raised about the future of pensioners' benefits, after Downing Street made clear that the Prime Minister's pledge to protect them applies only until the 2015 general election.
With health, schools and overseas aid protected in the spending review, Mr Osborne is thought to be coming under intense pressure to demand that the bulk of the savings come from the welfare bill, in order to avoid further cuts to areas like defence, criminal justice, the police, local government and transport.
Mr Cameron has already faced calls to ditch his promise, made during the 2010 general election campaign, to preserve universal pensioners' benefits like the winter fuel allowance and free TV licences, bus passes, eye tests and prescriptions.
The payments were protected for the period of this Parliament in the 2010 Coalition Agreement, but Deputy Prime Minister Nick Clegg has suggested that wealthier pensioners could be asked to "sacrifice" some of their benefits to save money.
Mr Cameron made clear last month he intended to keep his word, saying: "I made a very clear promise at the election that we would keep the winter fuel payments alongside the other pensioner benefits as they were, and that's a promise I'm keeping."
Asked today whether the benefits will be protected in 2015/16 - the first few weeks of which fall before the scheduled election date of May 7 2015 - a Downing Street spokeswoman said: "We have done an awful lot to help pensioners, but clearly, speaking generally, there are some difficult decisions to be made."
She added: "What (the Prime Minister) set out in terms of benefits for pensioners that was set out in the Coalition Agreement for this Parliament, he absolutely stands by that.
"But broader decisions around the spending review have yet to be made."
Mr Osborne is due to announce spending settlements for 2015/16 by the end of June 2013.
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