Sam Galbraith has come under increased pressure

Under the "derived grades procedure", a pupil's exam performance is usually compared to the result that his or her school expected them to achieve.

A computer then compares that prediction with the accuracy of the school's estimates in the past.

The checks can mean grades being improved - or failures converted to passes - before the results are issued.

They can also highlight problems with marking.

But the SQA says that this year the checks were run in only a limited number of old-style Highers and Standard Grades because school predictions were not available.

They were not run at all for the new Highers.

Deepening fiasco

BBC Scotland education correspondent Kenneth Macdonald said the lack of checks "could be a factor in the widespread reports of pupils whose grades were unexpectedly poor and the anticipated large number of appeals".

The news is the latest twist in the crisis surrounding this year's exams results - three weeks after all Scotland's pupils should have learned their grades.

Two senior SQA officials left their posts on Wednesday as the fiasco deepened.

No new Highers results were checked

Head of operations Jack Greig, who was responsible for the collection and processing of results, was suspended pending a disciplinary inquiry and director of awards David Elliot resigned.

The moves came after it emerged that most secondary schools had been sent the wrong Standard Grade results.

There have been widespread problems with the Higher results and repeated opposition calls for Scottish Education Minister Sam Galbraith to resign.

Mr Galbraith will make a statement to the Scottish Parliament next Wednesday.

The SQA said 4,264 Standard Grade results would have to be rechecked because they were inaccurate or incomplete.

Some of the affected pupils could be in for a shock - as they could receive reduced marks.