The Association of Teachers and Lecturers is to seek a judicial review over changes proposed by the Government to teachers' pension arrangements.
The union has sought advice from Cherie Booth QC, wife of Tony Blair, the Labour party leader, over a letter sent out by the Department for Education and Employment on the conditions for staff taking early retirement.
The Government is aiming to halve the number of early retirements by making local authorities, schools and colleges responsible for the pensions costs. This has led to a flood of applications aiming to beat the March 31 deadline.
The ATL is to challenge the DFEE over a letter which it says lays down new preconditions for an employer to allow a teacher to end his or her contract and retire. The letter from the department's pensions section in Darlington says termination of a contract may not take place if the school intends to take that person on again and if the result of the retirement means a temporary amalgamation of classes or supply cover. The letter says a permanent full or part-time teacher must be in place to take over the class.
The March 31 deadline means that schools could find themselves short of staff in the summer term. It was expected that many teachers who had taken early retirement would return to finish the academic year on a short-term contract - the new conditions will not allow this.
Peter Smith, the ATL's general secretary, said: "The changes to the superannuation regulations are still up for consultation and if agreed need parliamentary approval. This letter makes a mockery of the consulation process and is contemptous of parliamentary process. These conditions give a wholly new definition to the conditions for early retirement and are, in our view, perverse."
He accused the DFEE of cynically moving the goalposts and said the Secretary of State was exceeding her powers. The consultation period ends on January 19.
In a letter to The TES, Richard Hall, a North Yorkshire county councillor, says a substantial number of teachers have enquired about retirement, including more than 30 headteachers. He said: "I have also heard of the whole of a senior management of a large secondary school enquiring."
A survey by the National Association of Schoolmasters Union of Women Teachers predicts that applications for early retirement could double and up to 26,000 teachers could go.