It took 15 months to persuade the Government’s pension agency to back his case after twice being turned down while being off sick permanently. He eventually won his case in June.

Ministers, earlier this summer, announced a crackdown on public sector retirement on grounds of ill health because of escalating costs, prompting union claims that many teachers are now finding increasing hurdles in their attempts to leave the profession.

Around 200 teachers a year retire early because of medical conditions, Scottish Executive figures reveal. From February to May this year, 142 cases were processed, 107 accepted and 35 rejected. Psychiatric illness among secondary teachers tops the list of explanations with many complaining of illness for more than five years. Many are refused early exits despite psychiatric conditions.

Mr X said: “I was so sure I was going to get medical retirement. I would not have applied if I did not think I deserved it. I was quite happy teaching. It was the effect my teaching was having on the kids that made me apply.

“Eventually, I felt quite bitter about it. When the Government came out and said there were far too many teachers taking early retirement, I was quite enraged. It felt like your illness had to be terminal before you would satisfy the criteria of the pensions agency.”

He had had a heart attack and slight stroke in 1997, although he had never found the job particularly stressful. Three months later he returned to teaching but soon realised he had problems communicating because of stroke damage. “I did not think I was as effective as a teacher,” he admits.

By June 1998, he was told he would need a heart bypass operation and was placed on a year-long waiting list. He continued teaching but found the new intake less forgiving of his abrrations during the autumn session. “I found more problems with discipline and my explanations of simple questions were absolute rubbish. I stuck to required texts and lessons became more and more boring and problems worse,” he said.

Mr X went sick in March 1999 and applied for early retirement in April before his operation in June. He has been on six months’ full pay and six months’ half pay over the past year. He believes that the lack of classroom evidence about the negative effects of his teaching was the reason behind the first pensions refusal. “It’s the school culture. We were also having a change of rector at the time,” he said.

He reapplied, only to be refused again. “My own authority was very supportive and suggested I do the whole thing again. I realised I had to get evidence of teaching, so I got the school board, school, authority and my own doctor all on my side. The authority’s doctor said I should not go back to work.

“At no time did the regulations talk about your ability to teach.”

Government regulations for releasing pensions early state that “a teacher must be incapable by reason of infirmity of mind or body of serving efficiently as a teacher and despite appropriate medical treatment is likely permanently to be so”.

Mr X argues that these rules do not take account of his experience.

Barbara Clark, assistant secretary of the Scottish Secondary Teachers’ Association, said two-thirds of appeals were successful. “This suggests they should not have been turned down in the first place,” Mrs Clark said.

The union strenuously disputes claims that work-shy teachers are exploiting medical retirement rules in the Scottish Teachers’ Superannuation Scheme.

“Evidence from our members suggests that the reverse is the case and that teachers with serious health problems are finding it increasingly difficult to have retiral on medical grounds granted,” she stated.

“It is particularly difficult where the illness is psychological because there is often no clear prognosis, despite the fact that the teacher may be totally unfit to work. We have members who have run out of sick pay completely.”