More than 200 private schools are abandoning GCSEs in favour of a tougher, international version based on the old O level.

Bromsgrove and Rugby in the Midlands are among the independents that have chosen to adopt the more demanding qualification in some subjects. Called International GCSEs (IGCSEs), the exams were originally developed for those studying overseas.

But many public schools say the qualification is more academically rigorous and better preparation for A levels.

IGCSEs are also low on coursework and concentrate on assessment by examination. Other schools using IGCSE qualifications in subjects such as maths, science, English and history include Harrow, Millfield, St Paul's and Manchester Grammar.

Chris Edwards, headmaster of Bromsgrove School which uses the IGCSE maths course, said it was better quality than traditional GCSEs.

"In our sixth form we have 170 pupils taking maths and large number doing further maths, so we need a good GCSE course to prepare students for A level, university and beyond," he said. "IGCSE is a better course. It doesn't rely on coursework and concentrates on difficult subjects such as algebra and calculus, which the GCSE does not."

Although IGCSEs are not available in state schools - they are not accredited with the Qualifications and Curriculum Authority - exam body Edexcel said they are increasingly favoured by the independent sector.

The number of IGCSEs taken in maths, the most popular course, has risen more than threefold in a year from 2,200 last year to 7,500.

This is despite the fact that the qualification is not recognised in Government league tables and schools which adopt it risk slipping down the ranks.

"We designed IGCSEs for overseas students, so we were surprised when UK schools started adopting it," a spokes-woman from Edexcel said.

"But because they are exam-based, a lot of teachers seems to prefer them to GCSEs."

Coursework currently makes up about one-third of the regular GCSE maths course. The Qualifications and Curriculum Authority has already revealed plans to reduce the amount of coursework in both A levels and GCSEs.

But Geoff Lucas, secretary of the Headmasters' and Headmistresses' Conference which represents many public schools, said coursework was not the only issue making IGCSEs attractive.

He said: "Yes, I think the popularity of maths is a direct link to the amount of coursework, but when schools adopt IGCSEs in science, it may be linked to changes in the syllabus that some of our members believe are dumbing down the subject."

Mr Lucas called on the Government and exam watchdogs to allow state schools to use the qualification and for them to count on league tables.