'It seems science subjects are not only more highly represented [in grammar and private schools] but also considered very important,' said Sylvia Green, director of the research division at the exam board Cambridge Assessment, whose study is to be published tomorrow.

The report, 'A-Level Subject Choice in England', found that 33.3 per cent of pupils at grammar schools are now studying chemistry A-level and 27.7 per cent of those at private schools. However, at comprehensive schools the figure was far lower, at 14.8 per cent.

Similar trends occurred in the other subjects, according to the study of 6,500 pupils. Just over 21 per cent of pupils from grammar schools choose physics and 18.9 per cent at private schools, close to double the 9.8 per cent taking it in comprehensives.

In maths 31.3 per cent of grammar school pupils took an A-level, 39 per cent of those in private schools and 22.4 per cent of those in comprehensives, while in biology the figures were 32.7 per cent, 28.3 per cent and 20.6 per cent respectively. It is bad news for universities already under fire for recruiting too few pupils from the state sector.

'If these subjects are becoming increasingly the exclusive preserve of a subset of the education system, it is worrying for us and worrying for the country as a whole,' said Geoff Parks, director of admissions at the University of Cambridge. 'These are hugely important subjects essential for the UK economy.'

Parks said it was already tough to attract a good mixture of students from socially diverse backgrounds, and this would make it more difficult. Some pointed to the fact that more pupils at selective schools aspired to study subjects such as medicine and dentistry that required science A-levels. Peter Mason, principal of Stamford Endowed Schools in Lincolnshire, which comprises a private junior school and a private girls' and a boys' secondary school, said: 'If you ask students why they choose science, the main reasons are they enjoy the subjects, find them stimulating and need them for their future.'

The report comes just days after it emerged that the UK had slipped 10 places in an international league table for science teaching. But Derek Bell, chief executive of the Association for Science Education, said comprehensives were trying hard to attract pupils to the subjects. Chemistry, physics and biology were also seen as difficult subjects. He added: 'We don't want to reduce the standard but we do want to help students see that they can do it.'

The government says it is changing the way science is taught in an attempt to make it more popular. A spokesman for the Department for Children, Schools and Families said there would be more chance for pupils to study the sciences separately at GCSE and a pilot scheme that will introduce 250 science clubs for 11- to 14-year-olds.

'We will also double the number of science ambassadors - people with industry experience in science and engineering - to 18,000 by 2008 to work with teachers in schools to engage and enthuse young scientists,' he added.