Government plans to insist that at least one school or college in every town offers the international baccalaureate to sixth-formers will jeopardise standards, a conference will be told today .
A study for the Economic and Social Research Council says increasing the level of take-up of the IB would reduce average attainment levels for 18-year-olds.
The researchers, led by Professor Peter Davies of Staffordshire University, conclude that sixth-form performance would be reduced because those who opted for the IB would be unable to concentrate on their strongest subjects.
Under the IB, students have to study seven compulsory subjects, including science and a modern foreign language as well as the core subjects, instead of taking exams in just three subjects as traditionally happens with A-levels.
The research team, which will present its findings to the British Educational Research Association conference at London University's Institute of Education today, concludes that most students take rational decisions about their A-level subjects based on prior attainment up to GCSE.
The findings run counter to traditional arguments for promoting the IB which claim that most bosses want their employees to have a broader range of skills than just those demanded by the traditional three A-levels.
But Professor Davies's researchers conclude: "Evidence suggests that there is no relationship between a broader curriculum at A-level and subsequent earnings." They add: "If students' performance in advanced level subjects is strongly affected by their aptitude towards different subjects then reducing the scope for them to specialise in preferred subjects would reduce their average attainment."
They also said A-level students were likely to be more motivated because there is more chance of them pursuing a subject which coincides with their interests and ambitions.
Tony Blair pledged that at least one school or college in every town would offer the IB by 2010 and the International Baccalaureate Organisation says more than 100 schools in the UK will have taken it up by then.
A spokesman for the Department for Children, Schools and Families said yesterday: "Every young person should be offered a choice of rigorous challenging qualifications that suit their interests and abilities. That is why we are ensuring the IB is available in addition to A-levels from 2010."
- A study published today by the Joseph Rowntree Foundation says children as young as eight or nine are already writing off their educational chances because they come from poorer homes. The study found they felt " got at" by their teachers and lacked confidence.