A leading teachers' union is to issue a legal challenge to the government over its controversial pension reforms in a fresh blow to ministers' hopes of an end to the long-running dispute.

The NASUWT (National Association of Schoolmasters Union of Women Teachers) argued that the government should have embarked on a valuation of the Teachers' Pension Scheme to establish whether there was a problem with its viability and sustainability, before making changes.

The union has served a pre-action letter on the government Actuary's Department, the Secretary of State for Education and HM Treasury, setting in train a claim for a judicial review.

The basis of the legal action is that the NASUWT contends the Government had a statutory responsibility to conduct a valuation of the scheme.

The last valuation was conducted in 2006 and the union said regulations required a further valuation in a period not exceeding five years.

General Secretary Chris Keates said: "The Government has ignored our repeated requests to produce a valuation of the scheme.

"It is simply unacceptable and irresponsible for a Government to embark on changes which will have such a profound adverse impact on the financial future of teachers and their families without having evidence to demonstrate that a problem even exists.

"Not only is the Government failing to meet its obligations to teachers, it is failing in its duty to act in the interests of the public.

"It is however probably safe to assume that if a valuation would have provided evidence to support the Government's changes it would have produced it.

"The failure to provide the valuation has deeply angered teachers. The NASUWT has pledged to leave no stone unturned to defend teachers and their conditions of service and if this means recourse to legal remedy the union will pursue such action."

The NASUWT was one of a number of unions which lost a legal challenge to the government's changes to the indexing of pensions, with an appeal due to be heard next week.

Welsh teaching union UCAC said it was launching a consultation with members on the latest pensions offer from the government, ahead of a meeting of its executive next week.

The union said it had not signed the government's agreement as it considered the offer to be too detrimental to teachers, lecturers and to the education system itself.

General secretary Elaine Edwards said: "Members' views will be key to deciding on the next steps in this crucial campaign. The government's proposals will affect the vast majority of teachers in Wales whatever their age.

"That's why I'm encouraging members at all stages of their careers to let us know their opinion within the next week."

Negotiations over the pension reforms have still not been concluded, one of the main unions involved in the dispute said yesterday.

The GMB, which represents council workers, NHS staff and employees in the civil service, said a "lot more work" was needed before the talks ended.

Up to two million workers staged a one-day strike last November in protest at the pension changes, and some union leaders have raised the threat of more industrial action, possibly on 28 March.

The GMB's executive said in a statement: "The three separate negotiations GMB are involved in - local government, civil service and the NHS - are at a different stage and there is a lot more work to do to bring these negotiations to a conclusion.

"The executive agreed to give full support to the negotiating teams and agreed that GMB members, and no-one from outside the GMB, will make the final decisions when the negotiations are concluded and the final proposals are on the table."