Most teachers are over 40

And they say plans to outlaw age discrimination could amount to an attempt to force them to stay in the profession up to the age of 70.

Plans are already in the pipeline to make future civil servants, teachers and NHS staff work until 65 rather than 60 to get their full pension.

At the moment, most teachers retire at 60 with their full pension.

The proposed changes are due to be made by 2006 and would at first only affect new teachers and other new public sector workers.

But existing teachers will also be affected at some point after that, although the government has not yet said when.

The changes will only affect their future service but it will mean that anything earned after 2006 cannot be claimed until the worker reaches 65.

The net result would be that some teachers, or other public sector workers, would not be able to afford to retire at 60.

Younger staff would be particularly affected.

Bitter strikes

Mike Paget, who teaches in a school for children with special needs, is very worried.

"There's no doubt teachers are angry at this proposal," he told BBC News Online.

"Those teachers like myself who have faithfully paid into the pension fund for nearly 30 years are now being told 'Sorry - work another five years or lose 25% of your expected pension'."

"The same proposals in France have led to bitter strikes - the same may happen here.

"It won't be good for the teachers or their pupils to try to keep them in the job when they no longer want or feel able to do it."

People shouldn't be compelled to retire but shouldn't be compelled to stay in work either Sue Johnson, ATL teachers' union

The government announced on Wednesday that it planned to ban employers from forcing workers to retire below the age of 70.

The action is being taken, the government says, to combat age-discrimination, but teaching unions say it could also put pressure on them to keep working.

Sue Johnson, the head of pensions at the Association of Teachers and Lecturers (ATL), says her post-bag has doubled recently, as concern about the changes has grown.

"People are very uneasy about it, especially as the government won't say when existing staff will be affected.

She said: "We welcome the flexibility this gives so people can work beyond 60, but teachers should not be forced to work until they're 65, when they can draw their pension."

"People shouldn't be compelled to retire but shouldn't be compelled to stay in work either."

About two thirds of teachers in England are over 40, so many are already planning their retirement.

A spokeswoman for the biggest teaching union, the National Union of Teachers, said it would be wrong to force people to stay on.