The controversial grade boundaries were revealed after national test results showed that more than a quarter of children aged 14 also failed to meet the expected standards in maths and English.

Last night critics called for a review of the public examination system, claiming that the results were misleading and indicated a “progressive lowering of expectations”.

The revelation that pupils scoring as low as 16 per cent could be awarded C grades in one Edexcel maths GCSE paper and an A* grade with only 47 per cent in an AQA business studies paper was made after an investigation by The Times Educational Supplement.

But the astonishingly low marks for top grades were not isolated.

The analysis also revealed that pupils could achieve a C grade with 20 per cent in AQA GCSE maths, 25 per cent in classical Greek or 28 per cent in physics.

At Edexcel C grades at GCSE were awarded for 32 per cent in a French paper, 36 per cent in German and 39 per cent in religious studies, where administrative staff were used to speed up the marking processs.

Alan Smithers, director of education and employment research at the University of Buckingham, said that marks showed that grades now being awarded were “downright misleading”.

“Grades are to summarise a pupil’s achievements for the next stage of education and employment. If we’re giving such misleading messages, it’s no wonder that teachers and employers are expressing concern,” he said.

Claire Ellis, a spokeswoman for AQA, insisted that standards were kept the same year on year.

“We try to make papers comparable, but sometimes they are unexpectedly difficult,” she said. “If someone’s achieved an A* this year, it must be the same as an A* last year.”

John Dunford, the general secretary of the Secondary Heads Association, said that teachers and employers had to rely on the Qualifications and Curriculum Authority to maintain standards.

“Some papers are harder than others and therefore grade boundaries will obviously vary from year to year, although 16 per cent does seem extremely low for a grade C,” he said.

However, Professor Smithers said that since grades were no longer awarded as a proportion of the numbers taken, examiners have struggled to define an A grade.

He said: “Now a set of complex procedures has been devised that seems to produce absurd results. We really need to go back and review the whole system.”

The criticism, which further raises pressure on ministers over exam standards, came as statistics released by the Government showed that while overall test results for 14-year-olds had improved, more than a quarter of boys and girls were failing to reach the expected level in English and maths.

GIRLS ON TOP, BUT A QUARTER FAIL

Pupils achieving Level 5 or above in Key Stage 3, 2005

English 74% (girls 80% boys 67%)

Reading 68% (girls 75% boys 61%)

Writing 76% (girls 82% boys 70%)

Mathematics 74% (girls 74% boys 73%)

Science 70% (girls 70% boys 69%)

ICT teacher assessment 69% (Girls 74% Boys 65%)

Explore Education

• Schools

• GCSE & A Level Results

• Good University Guide

• Sunday Times University Guide

• Student

• Young Times

Times recommends

• Half of school authorities infringe admissions code
• Chair of group of top independent schools quits
• UK universities lose out to rivals