More than half a million civil servants may be asked to pay more towards their pensions, it has been reported.
Civil servants must pay more, Hilary Armstrong allegedly said
The Cabinet Office said the government was not reneging on the public service forum agreement - a deal reached last year on civil service pensions.
Key civil service union, PCS, said any attempt to "row back" on that deal would meet a "very robust response".
The Sunday Times said the plan was revealed in a leaked paper from Cabinet Office minister Hilary Armstrong.
The paper from Ms Armstrong, seen by the prime minister and the Cabinet, proposes that civil servants pay more towards their pensions.
She warns that the plan would be deeply unpopular and asks the government if it is prepared to stand firm even though there is a risk of industrial action.
Last year, the government agreed that new entrants to the civil service would retire at 65 like workers in the private sector.
Civil servants currently in the pension scheme are able to retire at 60, but can retire later if they wish.
There are discussions with unions as to whether the pension should be calculated on an average salary over the length of a person's career, or whether it should be based on final salary.
Mark Serwotka, general secretary of the Public and Commercial Services union, said unions were due to meet cabinet office officials on Tuesday when, it was feared, a proposal could be tabled.
He said: "Any moves to row back from what we have agreed would - as Hilary Armstrong recognises - be met with a very robust response from ourselves.
"What the government has got to remember is that the average civil service pension is £4,800 a year.
"So what we are talking about is people who are on low pay, with low pensions, who are effectively going to be asked to take a pay cut if they are going to be asked to contribute more."
A Cabinet Office spokesman said: "We don't comment on leaks, but any proposals on civil service pensions would of course be discussed in the usual way through the civil service unions and the Public Service Forum."
Lib Dem pensions spokesman David Laws accused the government of sending out "conflicting" messages.
"It is clear there is a growing gap between pensions in the private sector and many public sector schemes, which are increasingly being subsidised at taxpayer expense," he said.
"Instead of dealing separately, and often inconsistently, with different public sector schemes, the Government should establish an independent commission to inform the debate and make proposals."