The steady increase in exam pass rates over the past decade may have more to do with a rise in the number of middle-class teenagers than the quality of students or teachers, according to the Government’s exam quango.

This week’s A-level results showed the 17th successive rise in the pass rate. Some critics say this is because today’s exams are less rigorous, but an analysis by the Qualifications and Curriculum Authority found three alternative causes.

Officials identified improved teaching and clearer information about exams, as well as a 20 per cent rise in the proportion of teenagers from higher income homes since 1989.

A QCA spokesman said: “Pupils from middle-class families traditionally do better in exams so, with an increase in their number, we might expect results to get better overall.”

However, an academic who has studied the impact of birth rates and social class on exam performance believes that UK demographics mean that the results will soon level out.

Professor Roger Murphy, director of research at Nottingham University’s school of education, said that the rise in the proportion of middle-class births in the Sixties and Seventies had had a knock-on effect in exam results.

But this proportion was now falling as the number of children born to working-class families increased.

Professor Murphy said: “There is a 16 to 18-year time lag. But since 1977 the birth rate has been picking up again, particularly in social groups, such as working-class families, where there had been a pronounced fall.”

This year’s results also revealed that weaker candidates turned their backs on A-levels in favour of AS exams.

Although there were 1.1 per cent fewer 18-year-olds this year, 2.7 per cent more students sat AS-levels this summer - the same standard as an A-level, but half the content. However, these students did not do as well as last year’s with fewer than one in five getting an A or B grade compared with more than one in three A-level candidates.

Dr Ron McLone of the A-level boards’ general council, said: “Some candidates, especially those on modular courses, are deciding they cannot cope with A-level and want to take the AS instead. These are likely to be the weaker students.”

The drop in physics, chemistry and biology A-levels was particularly worrying he added. Only the most determined science students were taking these A-levels, said Dr McLone. As a result, there were fewer entries but more top grades.