The Government has insisted it is not planning to renege on a deal on civil service pensions reached with unions last year.
But it said arrangements for civil servants, which will allow existing staff to retire at 60 on pensions worth twothirds of their final salaries, must be "sustainable".
Unions reacted with concern to a leaked letter from Cabinet Office Minister Hilary Armstrong, which appeared to indicate the Government was shaping up for a showdown with Whitehall over the pension deal.
According to the letter, obtained by the Sunday Times, Ms Armstrong suggested that civil servants should be asked to contribute half of the cost of any future increases in the cost of their index-linked scheme, and that employers' contributions should in future be capped at 20 per cent.
The paper calculated that this could lead to Whitehall mandarins on #50,000 a year eventually being asked to pay #2,000 to #3,000 more annually in pension contributions.
Ms Armstrong warned fellow Cabinet members that the recommendation would be "very unpopular" with unions and urged them to be prepared to "stand firm" in the face of possible threats of strike action.
Mark Serwotka, general secretary of the Public and Commercial Services Union (PCS), said Ministers could expect a "very robust" response from unions if they tried to go back on a deal thrashed out with former Work and Pensions Secretary Alan Johnson at the Public Services Forum last year.
Unions are due to meet with officials from the Cabinet Office tomorrow and believe a Government proposal may be tabled then.
Under the controversial PSF arrangements, it was agreed that new entrants to the civil service would retire at 65, like workers in the private sector.
But Mr Johnson was widely criticised for leaving expensive pension provisions for existing civil servants unchanged, with retirement at 60 on two-thirds of final salary.
Mr Serwotka said: "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."
Yesterday, the Cabinet Office issued a statement saying: "The Government stands by the deal negotiated between Alan Johnson and the public service unions some months ago."