Headteachers today vowed to disprove suggestions that schools over-marked this summer's GCSE English papers as the row over the fiasco continued.

Two leading unions announced that they have asked their members to provide evidence to show teachers did not inflate marks to ensure pupils achieved higher grades.

Around 600 pupils in Hampshire and 136 in Southampton were left with lower grades because of the fiasco, despite doing at least as well as others who took tests early.

The city council is one of a number of local authorities taking legal action over the debacle.

A Report by England's exams regulator, Ofqual, into the chaos found that teachers were guilty of ''significantly'' over-marking papers amid pressure to produce good results.

Chief regulator Glenys Stacey warned that teenagers were let down this summer by an exam system that is abused by teachers.

In the wake of the report, one headteacher, Kenny Frederick of George Green's School in Tower Hamlets, East London, challenged Ofqual to produce evidence of its claims of over-marking.

She published feedback given to her school by exam board moderators which verified and praised its marking procedures.

The National Association of Head Teachers (NAHT) and the Association of School and College Leaders (ASCL) today said that they are urging their members to follow suit, and publish their own moderation reports.

Together, the two unions represent more than 45,000 UK school leaders.

Exam board moderators check a random sample of pupils' work each year to ensure standards are maintained.

Ofqual has said that its investigation into this year's GCSE English had found evidence of over-marking, mainly within acceptable ''tolerance'' levels, which would not have been altered by moderators.

But this still had an impact on grade boundaries, once all results and marks were looked at.

NAHT general secretary Russell Hobby said today: ''Last week, Ofqual suggested that it was correcting a fault caused by teachers, yet there appears to be little, if any, evidence that this was the case.

''If the exam boards themselves are confirming in their moderator's reports that teachers' marks were accurate, perhaps we can close down this particular diversion and get back on track.

''Ofqual must now accept its responsibility and start working with the profession to ensure immediate restitution for pupils caught up in the saga and to find ways to ensure that a similar mess does not happen again.''

ASCL president Mike Griffiths said: ''I've been appalled by the attack on the professionalism and integrity of the English teachers in my school and other schools around the country.

''Ours is a high achieving school with skilled and experienced English teachers who pride themselves on fair and accurate assessment of students' work. The moderator's report backs this up, yet the grades were changed in the summer. We're convinced that this is the same scenario in hundreds of schools around the country.

''That is why we're joining with NAHT in encouraging schools to make their reports public.''

Mr Griffiths has published the moderation report for his own school, Northampton School for Boys, which says that it ''is to be congratulated on its approach to the new specification and the accuracy of the marks awarded to students.''

In a response to Ms Frederick last week, Ofqual said: ''You ask whether we have evidence of over-marking. We do, as we set out in the report.

''The issue is this: while over-marking was mainly within moderation tolerances, if a significant proportion of schools over-mark, even within moderation tolerances, there is an inevitable impact on grade boundary setting, as examiners set grade boundaries by judging the quality of the work.''

NAHT and ASCL are part of an alliance of schools, councils and professional bodies which is pursuing legal action over the problems with GCSE English.