Teachers who have been declared unfit to work in the classroom are being approached in a "desperate" recruitment drive to fill vacancies in key subject areas, the National Union of Teachers said yesterday.

Letters from the Training and Development Agency for Schools (TDA), the main schools recruitment body, have been sent to teachers who have left the profession, including those who have retired on the ground of ill health. Describing teaching as "great fun", the letters boast that teachers now earn more and work less hard.

One letter was sent last week to John Illingworth, a former primary school head who made news headlines two years ago when he broke down in tears at the NUT annual conference and said that he was leaving the profession because of mental illness brought on by workplace stress.

Mr Illingworth, a former NUT president, said that he found the letter outrageous in its lack of sensitivity towards mentally ill colleagues and in its misleading claims over teacher pay and workload.

"I was forced to leave teaching two years ago because of mental illness," he told the union's annual conference in Manchester yesterday, adding that he had been declared "unfit to teach".

"I take that letter as a joke. But there are some very ill people out there who have left teaching and are still very ill.

"This letter could be extremely damaging to their health. It is outrageous that a government agency is sending out such letters to ill teachers."

He questioned why the agency had not found out which teachers had left the profession owing to mental health problems, adding that he would not be surprised if the letters had been sent to teachers who had died.

Mr Illingworth, originally a maths teacher, suggested that the agency could be writing to retired teachers of shortage subjects. Although there is no overall teacher recruitment crisis, there are shortages of maths, science and modern language teachers.

He read delegates extracts of the letter that he had received from Graham Holley, chief executive of the agency, claiming that a lot had changed over the past two years.

"Salaries are much better. Teachers are on average earning £10,000 a year more now than they did 10 years ago.

"The number of teachers working part-time has increased and the workload has improved, with teachers saying they spend significantly less time working at home," the letter said.

But Mr Illingworth contested these claims. "This isn't a half-truth. It isn't even a quarter-truth: it's damned lies," he said to applause.

Starting salaries for graduate teachers had increased by about £6,000 since 1997, and, in real terms, teacher salaries were less than two years ago, he said.

The latest survey on primary teacher workload, published last week by Cambridge University, showed an increase in average weekly working hours by two hours to 56 hours.

"We shouldn't be trying to encourage people into teaching on the basis of lies because, if we do, half of them will leave in the first three years of teaching. I know there's a crisis among teachers. That's why desperate measures like this are being taken. But the answer to that is to reduce teacher workload, improve our pay and keep us all in the job," he said.

A number of delegates approached him after his speech to say that they knew of similar letters being sent to NUT members, including those with mental ill health. It appeared that the Teachers Pensions Agency had passed to the TDA the names and addresses of teachers who had left the profession - something that the NUT said it would investigate. About 12,000 teachers return to the profession every year, joining a workforce of approximately 440,000 in England. But between a third and a half of teachers leave within five years of starting work.

A TDA spokesman said that it was actively encouraging qualified teachers to return to the profession. "Pay progression opportunities and flexible working arrangements have significantly improved over the last five years," he said. "Teachers are now also supported by an increased wider workforce, which frees up their time to do what they do best, which is to teach."