Professor Caroline Gipps, deputy vice-chancellor of Kingston University, said it was "easier" to study in single-sex schools, as working hard and asking questions were "not done" in front of girls.

Also a member of the Qualifications and Curriculum Authority's advisory committee, which regulates exams, Prof Gipps said there was "quite a powerful peer group factor" involved.

She added: "In single-sex schools they are likely to be less anxious about working hard and asking questions because this is not seen as being done in front of girls.

'Quite extraordinary'

"For boys in single-sex schools, it is much easier to follow and be good at subjects like English.

"The entirety of English entry at A-level is independent and grammar schools. Virtually no boy in mixed comprehensives does English up to A-level. It is quite extraordinary."

Speaking at the annual meeting of the Headmasters' and Headmistresses' Conference in Dublin, she stressed this was not a new phenomenon, although it had been largely overlooked.

This year's A-level results showed boys accounted for less than a third of the total entries in English, the same as in 2002.

Provisional figures from the Joint Council for General Qualifications showed there were 55,451 female entries in 2003 and 23,295 from males.