Prospect said the emergency plans were unlawful and undeserved and a breach of election promises made by both governing parties. It pledged to fight them in the courts and the House of Commons.

"Six hundred thousand staff are affected, of whom tens of thousands potentially face redundancy through no fault of their own," said Deputy General Secretary Dai Hudd.

"The Government wants to rip up their rights in a way that would do justice to King John. But it will not work. These rights are protected by law and bullyboy tactics will not get round that fact."

The plan announced by Francis Maude, Cabinet Office Minister, is to include the changes to the Civil Service Compensation Scheme in a 'money bill' that would make short-term cuts to the cost of the CSCS. Longer term, it will put down primary legislation to amend the 1972 Superannuation Act to remove the accrued rights of civil servants from legal protection.

Dai Hudd said: "We feel justified in opposing these measures as before the election ministers of both parties pledged to respect the accrued rights of staff. We feel strongly that the government should revisit the agreement we signed five months ago for the CSCS to be amended. Given the scale of the government's plans, those changes would save at least £500m for the public purse in the next three years. Isn't that the path a responsible government should pursue?"

In Prospect's view, the procedure envisaged by the Government for changing the CSCS would be an abuse of House of Commons rules, which require money bills only to be used for revenue-raising measures, to be scrutinised by the House of Lords and ultimately to be decided by the Speaker of the House of Commons.

The Government's intention is to cut maximum compulsory redundancy payments for civil servants to 12 months' pay and to limit payment for voluntary redundancy to 15 months' pay. The standard for the public sector - including teachers, nurses, police, local authorities – is two years' pay.

In March 2009, Philip Hammond, then shadow Chief Secretary to the Treasury, said: “All pension rights that have already been accrued should be protected.” In April 2010, Vince Cable, then shadow chancellor said: “We will not make any changes to pension rights that have already been accrued.”