Head teachers' leaders have written to the education secretary warning that next week's AS-level results could present a misleading account of achievement.

The National Association of Head Teachers says the publication of the exam results, expected next Thursday, could present an "inadequate and distorted picture".

Full text of letter

Heads are concerned that the complexities of the new AS-level qualification will not be made clear and results will be open to misrepresentation.

"I can appreciate your department's dilemma. The fullest possible publication of AS-level results on the 16th will avoid charges of "cover up" or "lack of transparency", says Mr Hart's letter to the Education Secretary, Estelle Morris.

"But it is essential that next week's publication is accompanied by a very definite 'health warning'."

Using results which were not fully representative "will lead to an inadequate and distorted picture which will be remarkably unhelpful to the new examination", he said.

'Cashing in' exams

The new qualifications, intended to widen the curriculum for sixth formers, are not treated in the same way as traditional A-level and GCSEs.

Instead students have various options.

They can "cash in" the qualifications to get a certificate and use them in applications for university.

Or they can decline to accept the qualifications - which in effect nullifies their sitting of the exams.

If they have cashed them in but want to retake them in January or June of their second year to try to improve their results, they must retake all three units.

But if they have not cashed them in they can retake any or all of the three units.

This depends in part on whether the AS is forming the first half of a full A-level or is being used in its other role as a qualification in its own right.

School entries

There is also a further confusion over the intentions of schools, as well as individual candidates, which have opted not to "cash in" results.

Heads say that if a large number of candidates and exam centres are not included in total results, any publication of an overall figure for achievement will be flawed.

In response, the Department for Education and Skills says that it "has made it clear that AS-level results to be announced will not show the full picture".

"They will not show entries from students who have not cashed in, or who plan to sit AS-levels in their second year of study.

"It is for individual schools and head teachers to decide when pupils sit their exams and when they cash them in," said a spokesperson.

Bumpy ride

The AS-levels have had a difficult first year, with complaints over exam overload and accusations that their introduction had been a "shambles".

But a review of the changes to the exam system, announced by Estelle Morris, is expected to that mean sixth formers will take fewer exams in future.

Announcing changes for next year, the education secretary said: "All new exams take time to bed in, but the new AS has had more than its fair share of problems."

Scotland's exams body has admitted that a "handful" of pupils may receive incomplete exam results next Tuesday.

The Scottish Qualifications Authority (SQA) has re-checked 871 exam results but the total number of unresolved cases remains at 1,609.

The organisation has also outlined emergency plans for dealing with any outstanding cases, which include providing a full explanation of reasons behind the decision not to issue full certificates before Tuesday.

Some 135,000 candidates entered 732,000 national qualification exams this summer.