Lawyers for the lecturers' union NATFHE, which has 70,000 members in the Teachers' Superannuation Scheme, are preparing a bid for judicial review.
The union claims Education and Employment Secretary Gillian Shephard exceeded her powers when she proposed transferring the cost of early retirement from the pension scheme to employers.
Leaders are also challenging the Government's plans to prevent those taking up ill-health retirement after the beginning of April from teaching part time.
NATFHE General Secretary John Akker said: "The Secretary of State is asking employers to find the funds for a scheme for which they have never received any contributions, while their employees have paid in to the TSS for years.
"Our members feel that they have been cheated. The TSS handbook advised them to join the scheme because of the redundancy benefits on offer, with immediate payment of pension if they were made redundant from age 50.
"Lecturers deserve a decent pension, not the prospect of being pushed onto the dole queue and into poverty."
Lawyers acting for NATFHE wrote to the Department for Education and Employment this week, and warned that legal action will follow if officials do not respond by March 21.
The NATFHE action is the second legal challenge to Mrs Shephard's proposed pensions reform.
In January the Government was forced to withdraw a controversial form asking for details of all early retirements since October, after legal action by the Association of Teachers and Lecturers. And last month Mrs Shephard postponed the clampdown on early retirement until August, amid warnings that many teachers made redundant in their 50s will have to wait until 60 before getting their pensions.
NATFHE leaders expressed concern that pension changes were being introduced at a time of cuts and a rise in demand for early retirement because of increasing workloads and stress levels.