Lisa Holland: Inspectors claim they have evidence that most head teachers find reports fair and helpful

Commissioned by the National Union of Teachers, it suggests that headteachers reject the proposition that inspections are "supportive and motivate staff".

Instead, they speak of stressed staff and disruption, with one respondent describing the inspectors' visit as "the most intimidating experience of my life - the whole effect on upon staff and pupils was negative".

The survey, based on replies to a questionnaire by 1,250 heads and deputies at schools in England, found 79% disagreeing or strongly disagreeing with the suggestion that inspections helped to motivate staff.

Staff "can be demoralised by the inspection teams"

But there was better news for the Office for Standards in Education inspectors in other aspects of the survey, with opinion evenly divided over whether the inspectors' judgements were fair and accurate.

Even though individual testimonies from the NUT's respondents describe inspectors as "aggressive" and "abrasive", the survey shows that almost half of the headteachers believed that the inspectors "established a professional dialogue with teachers".

More worrying for the inspection process was the large majority who said that they did not believe that an Ofsted inspection had led to their schools improving.

The anonymous comments from headteachers that accompany the union survey present a bleak picture of schools shell-shocked by the inspection process.

Doug McAvoy: "There are better methods we could use"

One respondent said: "The headteacher at our school (I am now acting headteacher) has been off school for the past 14 months with post-inspection stress which resulted in a complete nervous breakdown."

In another report, a headteacher said that "the Ofsted inspection resulted in an overload of work, demoralised and stressed staff and had a detrimental effect on children's learning ... as a manager I found it difficult to remotivate staff and believe the Ofsted experience has a negative effect on teaching and learning".

The NUT's General Secretary, Doug McAvoy, said the survey showed that the stress and extra work created by inspections could not be justified.

The NUT claims that pupils do not benefit from the inspection process

"Both in their responses and comments, heads and deputies have given a thumbs down to the present system of inspections.

"Overwhelmingly they say that the ends do not justify the means. Inspection does not lead directly to school improvement."

The Shadow Education Secretary, David Willetts, defended Ofsted's record, saying it had been of "enormous benefit" to teachers and pupils.

He claimed it was the target of "a hostile agenda from some in the education world" and called on Education Secretary David Blunkett to "robustly defend" his inspectors.