The age at which people in England get a free bus pass could go up from 60 to 65 sooner than previously planned, if new cost-cutting plans are agreed.
But government sources told the BBC they thought it was unlikely to happen.
Labour wanted to push the age of eligibility up to 65 by 2020, but the Department for Transport has suggested bringing this forward as part of the coalition government's budget cuts.
Currently, the department spends £1bn a year on free travel for the elderly.
The Department for Transport's idea is among its initial proposals for the budget cuts being submitted to the Treasury by all departments later.
Most departments have been told to go through their budgets for savings of between 25% and 40%, with the government's public spending review expected to be completed in October.
At the moment, anyone born before April 1950 is eligible for an annual bus pass giving free off-peak travel anywhere in England.
Since April, however, the age of eligibility for concessionary travel has begun gradually increasing from 60 - to reach 65 for both men and women by the end of the decade.
This was an idea announced by Alistair Darling in March in his final Budget as chancellor.
The move was predicted to save £60m annually from the free travel scheme.
The exact details of how quickly the coalition government would bring in the age increase to 65 have not been disclosed.
Deputy Prime Minister Nick Clegg said: "We're not going to scrap free bus passes but clearly we're going to review how it operates.
"But it's far, far too early to suggest we are going to do one thing or another, because this is all the kind of thing we're going to have to look at in the round as we try to balance the books over the autumn in the comprehensive spending round."
The DfT also insisted the government was "committed to protecting free bus travel for older people" and the proposal was "being examined as part of the spending review".
A source told the BBC all available options were being considered for the spending review.
But later, government sources cast doubt on the proposal, saying it was part of the drastic scenario of making cuts of 40%, rather than the 25% cuts which were more likely.
The source also said by denying free bus travel to pensioners, the government could be leaving itself open to legal challenge.
It is understood in his submission to Chancellor George Osborne later, Transport Secretary Philip Hammond will say he has decided to stick with Mr Darling's plan.
It comes after Mr Cameron accused Labour of "pure and simple lies" when they accused him of planning to scrap bus passes during the general election campaign.
The prime minister's official spokesman said David Cameron was committed to protecting free bus passes for older people but the department would look at ways to make savings for greater efficiency and better administration.
He denied that David Cameron was reneging on an election promise.
But shadow transport secretary Sadiq Khan said David Cameron owed the public an apology.
"His mock outrage during the general election campaign has been shown up as a cynical and misleading act, and his manifesto commitment to protecting concessionary fares as a sham.
"But more importantly, this news will cause anxiety for hundreds of thousands who could be affected - having misled them during the election campaign, David Cameron owes it to them to come clean now and confirm unequivocally what his plans are for free bus passes."
In his budget last month, Chancellor George Osborne said departments would face average cuts of 25% when the spending review was completed.
Health and aid spending will be ring fenced, while defence and education will face cuts of between 10% and 20%.
Michelle Mitchell, charity director of Age UK, which combines Age Concern and Help the Aged, said: "Although we understand that tough decisions need to be made in today's economic climate, it must not be ignored that free local bus travel is a lifeline for many older people.
"It helps people to remain independent and access local amenities and services, and can be an important way to keep in contact with friends and family, helping to tackle social isolation.
"Given the value many people in later life place on them, we would urge the government to think very carefully before making any changes to eligibility for free bus passes."