Figures obtained by Don Foster, the Liberal Democrat education and employment spokesman, reveal that three years ago 337 heads retired prematurely; in 19945 the figure was 559. The number of classroom teachers leaving on ill-health grounds has risen by a third - taking the total to nearly 4,000.

And Mr Foster said: “There is a worrying increase in demoralised teachers suffering stress-related illnesses, taking early retirement and bowed down by constantly changing demands and lack of support.

“This Government is expecting more and more of teachers with less and less - the strain is clearly showing.

“We should not tolerate poor teaching. But to get the best out of our teachers we must give them the tools to do the job - books and equipment and decent buildings.”

The figures, obtained in a Parliamentary answer, cover the years 19912 to 19945 and ill-health retirements from maintained nursery, primary, secondary and special schools.