One of the UK's largest teaching unions has rejected the government's proposals for the reform of the teachers' pension scheme.

The National Association of Schoolmasters Union of Women Teachers (NASUWT) met to consider the offer made by the Department for Education (DfE), which was presented to teacher unions on 20 December last year. The national executive of the NASUWT "unanimously" endorsed the stance previously taken by the union's negotiators not to sign up to the government's "Heads of Agreement".

The decision comes after thousands of teachers downed their tools on 30 November in protest of the pension proposals.

Chris Keates, general secretary of the NASUWT accused the DfE of wasting "valuable time" and described the consultation process as a "debacle".

"[The DfE] failed to provide the necessary information on which meaningful discussions could take place and presented some potential changes which did not have Treasury approval.

"Unions were pressurised and threatened to sign up to a document when a final draft was not even available. Even when a document was produced as the final meeting was breaking up, the wording was changed unilaterally by the DfE overnight."

Keates said it would have been "completely irresponsible" for the union to sign up to the Heads of Agreement under the circumstances.

"These are critical issues which have profound implications for teachers, for the profession and for the education service."

Keates added she would be writing to Michael Gove requesting he meets with the union as a "matter of urgency".

A Department for Education spokesman said it was "disappointing": "The deal we set out before Christmas is a fair one for teachers and affordable for taxpayers. It means that teachers will still get a far better pension than the vast majority of people in this country - while we can keep long-term costs firmly under control.

"It's disappointing the NUT and NASUWT restated their position on last month's deal. We've already addressed many of teachers' concerns, particularly around early retirement. We are now ready to have detailed, technical discussions to reach a final settlement - but have made clear that the broad deal on the table is as good as it gets. Other unions have given a far more positive response and will be consulting with their executives later this month.

"Reforms to public sector pensions are essential - the status quo has never been an option. The cost to the taxpayer of teacher pensions is already forecast to double from £5 billion in 2006 to £10 billion in 2016 and will carry on rising rapidly as life expectancy continues to improve."