Winter fuel allowances, free bus passes and TV licences for the elderly could be cut after Downing Street warned the public to prepare for "difficult decisions”.
David Cameron has promised to protect the benefits until the current Parliament ends, in May 2015.
But Nick Clegg believes “millionaire” pensioners should lose their entitlement to save money for other priorities.
The future of the payments was thrown into fresh doubt on Friday after Number 10 suggested that they would be reviewed and senior Liberal Democrats called for them to be means-tested from April 2015.
Talks between the Conservatives and Lib Dems over a fresh wave of cuts have already begun.
Mr Clegg is said to be ready to call for means-testing pensioners’ benefits in the negotiations, if Tories call for more cuts to welfare.
The Chancellor will announce final public spending plans for the 2015-16 financial year by the end of June.
At a regular briefing for journalists in Westminster, a spokesman for the Prime Minister said: “We have done an awful lot to help pensioners but clearly, speaking generally, there are some difficult decisions to be made.”
The Downing Street spokesman suggested that Mr Cameron’s promise to protect pensioner benefits did not extend to the 2015-16 spending review: “Decisions beyond this parliament have yet to be made.”
Speaking later, a separate statement from the Prime Minister’s official spokesman appeared to contradict the earlier Downing Street comments.
“Pensioner benefits are fully protected for the entirety of this Parliament - including for the year 2015/16,” the spokesman said.
Downing Street said the promise was enshrined in the Coalition Agreement of 2010.
However, senior Liberal Democrats questioned whether the agreement would cover spending in 2015-16, which is currently being negotiated inside the Coalition and will be finalised in the months ahead.
Mr Clegg, the Libl Dem leader, has made it clear that he wants to cut the benefits for the wealthiest pensioners.
A senior Lib Dem source said payments to “millionaire pensioners” would be first in line for any cuts to welfare.
“This is something that should be looked at,” the source said.
A senior Tory said the Prime Minister would have to make a clear announcement on the policy ahead of the election, adding: “This is going to remain a live issue.”