Private schools will flout new age discrimination laws by continuing to force teachers to retire at 60, union leaders said this week.
The Association of Teachers and Lecturers said six schools had already told staff that they would not be able to work past the age of 60, even after European legislation is introduced in 2006 banning such compulsory retirement.
Thousands of private-school teachers are expected to be affected by the stand-off, according to the union.
Under the European Union's employment directive, companies will no longer be allowed to dictate when staff retire.
Private schools are currently free to draw up their own contracts for teachers, in which they set their own fixed retirement age. The ATL, which represents 19,000 private-school teachers, said the "overwhelming majority" force staff to quit at 60, unlike the state sector where teachers can work until the age of 65.
Speaking ahead of their independent schools conference in Birmingham tomorrow, ATL officials said some schools have told staff that they will maintain a compulsory retirement age of 60.
Jonathan Restell, the union's national official for independent schools, said: "We have already been informed about six schools where staff have been told they will still have to retire at 60. Our fear is that this is indicative of a much wider trend in the independent sector."
Mr Restell said: "Why is 60 so vital when for decades teachers in the state sector have been able to work beyond the age of 60, if they choose to do so?
"The union regularly advises members with good health and excellent records who are not allowed to carry on teaching. Sometimes they cannot afford to stop working, but more often than not they simply do not want to stop doing something they enjoy. Unfortunately until 2006 there is little they or we can do legally."
A spokesman for the Independent Schools Council (ISC) said ATL's claims were "unfounded" and said: "We have not heard of any school which would intentionally break the law and force staff to retire at 60 - the 2006 ruling will state you will not be able to enforce any retirement age."
At the moment most private schools use recommended contracts drawn up by the Association of Governing Bodies of Independent Schools. It is believed they will have to be redrawn.
The ISC said: "Most schools use a standard teacher contract which specifies retirement age at 60 and most teachers are happy with that."
The ATL has been campaigning for the abolition of compulsory retirement ages. It says the contracts of most teachers in private schools are not flexible enough and all teachers should be allowed to to work until65 if they want to.
Meanwhile, in the state sector, controversy is still raging over the planned increase in the normal retirement age from 60 to 65, with those retiring before 65 facing a loss of benefits.
Ministers have proposed sweeteners to soften the blow of having to work till 65, including a better deal for spouses.
But, as The TES revealed last week, the full range of improvements could cost teachers between pound;100 and pound;660 extra a year, which has angered trade unionists.
Joint union guidance on the proposals say they undermine attempts to achieve "long overdue" improvements in pensions.
ATL, along with the other major teacher unions, has opposed a compulsory increase in the normal retirement age.
Many private schools are members of the teachers' pension scheme - these schools may have to raise their compulsory retirement age in future to fit with the state sector.