More than 100 potential cases are being investigated by the largest classroom union, the National Union of Teachers, and a further 25 to 30 cases are being processed by the courts or being prepared for litigation, according to NUT solicitor, Graham Clayton.

The National Association of Schoolmasters Union of Women Teachers has 18 current stress claims pending and is getting up to two new cases a week, according to legal officer Mary Howard.

She said each one of the 18 claims involved instances of teachers having had at least two significant breakdowns due to work pressures, or protracted anxiety problems which were not tackled by their employers.

Mr Clayton said most of the NUT cases concerned workload and the inability of senior managers to help.

He said: “I am not eager to be fighting a flood of cases. Not because I don’t want to but because we are looking for a cure not a remedy. This has demonstrated that there is a serious problem for the local authorities, governing bodies and the Government itself to tackle.

“It is not the fault of the individuals, it is the fault of the system. Senior managers have not been trained in how to deal with it and so teachers tend to get this dismissive attitude.”

Muriel Benson, an English and media studies teacher and head of year at Prenton girls’ high in the Wirral who had to work a six-day week because of lack of non-contact time, was told she was a victim of her own success and should dissuade pupils from taking her classes.

Mrs Benson, who retired through ill-health in 1996, eight years after first making her complaint, won a pound;47,000 out-of- court settlement from Wirral Metropolitan Borough last week.

Although this was the first case brought by the NUT, a previous out-of-court settlement, also for pound;47,000, was secured by the NASUWT this spring for a teacher in his 50s who launched a claim against an unnamed authority in the North-west.

Ms Howard said the “target-setting” culture in schools was clearly having a stressful effect on teachers, but the Government had begun to recognise the problem by acting to try to reduce bureaucracy and by supporting the new stress helpline for teachers. The helpline has received hundreds of calls since it was set up by the Government a month ago.

But the Benson settlement is not necessarily a breakthrough.

Ms Howard said: “It’s only a settlement: it does not set any precedent in case law on which we can argue for other cases.”

This ideal scenario for the unions, in which clear ground rules would be established for teachers suing for stress, was precisely what local authorities were trying to avoid in agreeing to out-of-court settlements, she added.

A Northumberland social worker, John Walker, became the first person in Britain to successfully sue his employer in a stress-related case in a landmark 1994 ruling.