Students of the same ability taking single science GCSEs get more top grades and go on to secure better A-levels, according to Professor David Jesson of York University.

The findings seem to undermine the common belief that separate sciences are harder than combined courses worth two GCSEs.

Only 5 per cent of pupils at comprehensives take separate sciences, while more than 2,000 schools do not even offer individual courses.

The Confederation of British Industry has demanded urgent action to reverse a long-term decline in the study of science. It is calling for 40 per cent of pupils to take separate sciences.

Professor Jesson’s research compares the exam performance of the top 40 per cent of pupils taking separate and combined sciences.

He divided them into five ability categories. In each one, pupils taking separate sciences outperformed those taking combined courses.

For example, in the lowest ability group, 9 per cent of students taking combined science achieved an AA* at GCSE, while 20 per cent of those taking separate sciences received an AA*.

In the next ability range, 15 per cent of pupils taking combined sciences received an AA* compared with 27 per cent taking separate sciences.

“There’s an encouraging advantage to young people taking single sciences compared with double sciences,” said Professor Jesson.

“This analysis compares like with like, and the results could not be clearer. This will shake some comfortable assumptions. If every child matters, are we giving them genuine choices?”

The Government is encouraging independent schools to partner state schools to help improve their science teaching. But less than a third - just 31 per cent - of private school pupils took single sciences last year, the research shows.

Grammar schools had the highest rate of pupils taking single sciences, at 35 per cent.

Marianne Cutler, head of curriculum innovation at the Association for Science Education, said: “Children taking separate sciences will probably have more lessons allocated to them on the timetable than those doing double.

“A lot of the results will depend on individual pupils. But the expertise and background of the teachers, the facilities at the schools and their resources all have to be taken into account.

“Many schools do not have specialist physics and chemistry teachers. Where they do, they have a better chance of getting better results. Teachers’ expertise and experience are the most important thing.”

She said that schools should explore the range of courses available and choose the ones most appropriate to their pupils and resources.