Out of 22 cases heard since tough new rules came into force last April, the National Association of Head Teachers says it has won 17, some of them on second appeal. Another 28 have still to be heard - the union has just taken its 50th case on to its books.

The NAHT also plans to step up its campaign against the rules, which make it harder to retire early. The union condemned the changes as “inhuman” at its conference last month.

Mike Beard, assistant secretary responsible for salaries and pensions, said members already burdened with conditions which left them unable to teach were suffering unnecessary additional anguish because of the difficulty in obtaining ill-health retirement.

“The results give us hope, but we’d still prefer the criteria to be changed,” he said. “The individuals are being put through further stress by having to go through this process - for some of them, it’s not just once but twice.”

The new regulations, introduced by the Conservatives near the end of their final term, tightened up the conditions for early retirement. Previously, teachers needed to demonstrate they could not function efficiently. That often allowed them to return later to teaching, perhaps part-time. Now they must show they have been left permanently unable to teach in any capacity, full or part-time.

Cases are judged by medical assessors for Teachers Pensions, the private company that administers the profession’s superannuation scheme, on the basis of medical evidence from the teacher’s doctor or specialist.

“Specialists are saying to us ‘They don’t trust us. We know the individual patient but they don’t believe us,’’’ Mr Beard said.

Doctors warned that one applicant had a heart condition which could kill him if he returned to work. The TPA decided it was stress and threw the case out.

In another, the NAHT says, the assessors cast doubt on the existence of ME or chronic fatigue syndrome. They again maintained the teacher was suffering from stress and therefore not permanently unable to work.

Several successful appeals involve cases rejected outright because application forms had simple errors or the evidence was not sufficiently detailed. “Rather than saying come back with the proper evidence, they refuse it,” Mr Beard said.

Teachers can resign and preserve the pension earned so far. But they lose any enhancement - equivalent to up to six-and-two-thirds years of service - and the pension payments that they would have received before the age of 60.