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, it has been announced.

The NASUWT argues 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 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.