Some results have not yet been double-checked

As a trial this year, reading and writing parts of the national curriculum tests were sent to different examiners.

There was a delay in sending out the results to thousands of students, with some arriving after term ended.

The National Assessment Agency has apologised, but head teachers fear there may be a bigger problem.

Combining the results took longer than predicted, so that even an extended deadline was missed.

The problem primarily affected the Key Stage 3 national curriculum English tests - those taken by students aged 13 and 14, in their third year of secondary school.

Extension

The marking was done by the AQA exam board. The results were collated by the Data Collection Agency, part of the Pearson group.

A spokesman for the National Assessment Agency said: "The Key stage 3 English marking process was changed this year to improve marking quality.

"Markers opted to mark either the reading or the writing papers.

"The marking period was extended by a week to allow time to marry up the reading and writing marks and calculate an overall level for English.

"However, for some schools the process took longer than expected.

"We recognise that for some schools this made it difficult to report results to parents by the end of term, and we have apologised for the inconvenience this delay has caused."

Not only that, but it appears wrong marks were sent out to some schools.

Derbyshire teacher Gill Chesney-Green told BBC News Online: "The English Sats results in our school have been examined carefully - and there are a lot of mistakes in the addition of marks."

Many of the scripts were being returned, delaying the results for the school.

"Apparently this is happening on a large scale locally," she added.

Not checked

Part of the issue involves so-called "borderlining" - the process of double-checking the marks of students who are close to a higher test level.

To get the results out, scripts were sent back to schools without this extra check. Schools have been told they have until 9 September to query them.

The director of testing at the assessment agency, Jackie Bawden, said in a letter to head teachers: "This will ensure that no pupil is disadvantaged and also ensure that schools are not required to deal with this matter during the holiday period."

The agency said 170 schools had received late results, but the problem may be more widespread.

The Secondary Heads Association said it had received a larger response from its members on the issue than it had ever had before.

Deputy general secretary, Martin Ward, said: "The tone of the messages varies from angry to anguished to weary."

Common complaints included delays, poor communication and administrative errors.

'Faith eroded'

"But the most frequent and most significant is erratic marking - most worrying is that there are markers who have marked without regard to the marking scheme or indeed the students' work (for example giving all the candidates the same mark for a particular section).

"This implies a failure of the moderation process that raises doubts about the reliability of the awarded marks in general.

"I am concerned for the effect on students, who may be upset by disappointingly poor results, who may have false expectations raised by errors in the other direction, and who may be demotivated by a general cynicism about exams.

"What faith we may have had in these tests (little enough already) stands to be further eroded."

The delays might have some effect on the interim national results, due to be published in August.

The final Key Stage 3 English test results form part of one set of performance tables for secondary schools, published later in the year.

The pupils who took them are the first to have completed all three years of the government's strategy to raise achievement between the ages of 11 to 14.