The SNP have been told to come clean over plans to increase the minimum age for a bus pass
Last month's draft Budget revealed spending on the concessionary travel scheme will fall from £207.8 million to £198.3 million next year.
The Scottish Government has previously ruled out raising the eligibility age for a travel pass from 60 to 65.
But ministers are reviewing its "longer term sustainability" with soaring demand and an ageing population.
It is thought a consultation on hiking the age was due to be launched early this year – but has now been shelved until after May's councils elections.
A shake-up is unlikely to affect anyone with an existing National Entitlement Card.
Sources familiar with the situation said changing eligibility was the "obvious place to go" and warned "something's got to give".
Demand for free concessionary passes has risen since they were introduced in 2006, with costs increasing by a third.
Some 200,000 of the 1.2 million currently issued belong to those between the ages of 60 and 65.
Scottish Labour's transport spokesman Neil Bibby said: "The SNP should come clean about these plans now – keeping older people in the dark about the future of the bus pass would be an insult. "The free bus pass was introduced by the last Labour-led Scottish Government and it is a lifeline for many older people.
The alleged plan comes amid a £10million funding gap
"If the SNP are planning changes to it, then they should say so before the elections in May."
If the SNP are planning changes to it, then they should say so before the elections
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Scottish Labour's transport spokesman Neil Bibby
In her programme for Government this year, First Minister Sturgeon pledged free travel "will continue" for older people and the disabled.
But the document stated: "We will also examine with stakeholders options to safeguard the longer term sustainability of the concessionary travel scheme."
This was reinforced last month's Budget ahead of a new deal being struck to reimburse bus operators.
Spending on the concessionary travel scheme will fall from £207.8 million to £198.3 million
Last month when quizzed by MSPs over whether the entitlement would remain unchanged in the wake of the cash shortfall Transport Scotland's finance director Mike Baxter said: "For those who have the card, yes, absolutely." But he made no mention of future applicants.
This could leave Scots worse off than many parts of England, such as London, where the concessionary travel scheme starts at 60. However, figures have previously suggested around one-third of pensioner bus passes are being used by Scots who are still in work.
A Transport Scotland spokesman said: "Free travel for Scotland's older people and disabled people on local or Scottish long-distance buses will continue, through the National Entitlement Card, which we remain absolutely committed to. "We will be consulting with stakeholders soon on ways to protect the long-term sustainability of the National Concessionary Travel Scheme."