Most public sector pensions are funded from the public purse each year, rather than from investments. Like similar schemes across Europe, they are increasingly considered too expensive to maintain by parties across the political spectrum.

Unions have argued that any cuts should affect all public sector workers, including MPs, who enjoy the most generous pensions in the public sector.

Stephen Yeo, a partner in Watson Wyatt, said: "The government has used an artificially high interest rate to judge how much they will need to fund these pensions."

A Treasury spokesman said: "This is simply an acccounting issue and has no impact on the reality for taxpayers or workers now or in the future. We have published separately and in detail our long-term fiscal projections on how liabilities will actually fall for decades to come and have shown how the UK's fiscal position remains fully sustainable."

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