Some 400 judges have taken an unprecedented first step towards suing the Government over changes to their pensions that effectively amounts to a pay cut.

They have sent a letter to the Lord Chancellor, Kenneth Clarke, warning that if he proceeds with the controversial plans, they will challenge him in the courts.

The action by the judges, who have instructed the international law firm Clifford Chance to send the “letter before action”, gives ministers 14 days to respond before legal proceedings are launched.

It comes despite efforts by senior judges to negotiate behind the scenes to prevent the dispute escalating into what would be a ground-breaking legal action, pitching the judiciary against ministers.

A judge would have to be found who was regarded by all sides as as sufficiently impartial and robust to hear a case brought by his judicial colleagues.

This week the Ministry of Justice went ahead with laying regulations that will see judges make a contribution to their own pensions for the first time from April.

From April, judges who have not accrued full pensions benefits will pay 1.28% of their salary to fund their pension pot, a sum of around £137 a month for circuit judges in crown courts. The sum will rise in 2012 and 2014.

The Ministry of Justice said that the move will reap £7 million of savings to the taxpayer. For a crown court judge on a salary of £128,296 the contributions would amount to £1,642 a year.

But judges, who include district judges, High Court costs judges, tribunal and employment judges, are furious that the contracts on which they took up their posts are being changed.

They say that the changes are unlawful, unconstitutional — encroaching on judicial independence — and a breach of contract.

They say that taken over time, the cut to pay and pension combined could amount to £200,000.

Kenneth Clarke said in a written statement to Parliament that the rises in judicial contributions are in line with those for other public service pension schemes, aimed at saving £2.8 billion a year by 2014/15.

A Ministry of Justice spokesman added: “These changes will see judges contribute towards their own pension for the first time, creating up to £7 million in savings for taxpayers.

“Lord Hutton’s Independent Public Service Pensions Commission has concluded there is a clear rationale for public servants to contribute more towards their pension costs so they remain fair to taxpayers and employees, and affordable for the country — this includes judicial pension schemes.”

Judges ready to sue Government over pensions