The A-level is becoming seen as an exam for lower ability pupils in schools offering alternatives such as the International Baccalaureate (IB) and the Cambridge Pre-U.
Non-selective independent schools contacted by The TES that are running twin-track systems say that, although there are exceptions, A-level pupils have on average lower ability than those who opt for an alternative.
Millfield School in Somerset has enough pupils to run two history A-level groups alongside two history pre-U groups. Craig Considine, the school's head, said: "What we have found is that a number of our pupils have not quite ended up streamed but pretty much so. The more able kids have opted to take the Pre-U in comparison with the more straightforward A-level."
The news is another blow to the A-level's "gold standard" status. Its credibility has come under increasing fire following the advent of modular exams allowing retakes and spiraling numbers achieving "A" grades.
Last week a government commissioned report into science and maths education expressed concerns about a lack of maths content in maths and chemistry A-levels.
Geoff Lucas, Headmasters' Headmistresses' Conference (HMC) secretary, said he was unsurprised that more able pupils were opting for the Pre-Us - which offer more traditional, non-modular, linear courses.
He said there were parallels with when schools first started to move from A-level to the IB, which offers greater breadth and had tended to be viewed as "academically stronger".
But Mr Lucas said pupils' choices could be about their learning styles as well as ability.
John Newton, head of Taunton School, which offers the IB alongside A-levels, said: "I think the average IB student in our school has a higher than average set of GCSE results than an average A-level student. But the highest performing A-level student would be as able as the highest performing IB student. The IB is horses for courses, depending on your preference rather than ability."
Andrew Halls, head of the academically selective King's College School, Wimbledon, which has moved completely over to the IB from a dual system, said the A-level had become better suited to lower ability pupils.
"We have had a decline both in its content and its ability to differentiate (between pupils)," he said. "It has become a much bigger fig leaf. You can get a "B" at A-level in a way you couldn't 20 years ago, without being that good and without doing that much work."
But Christopher Ray, head of the selective Manchester Grammar, said his IB and A-level groups had near identical ability profiles. "It is a myth that the IB is for the brightest and the best," he said.
Geraldine Seymour from Cambridge Assessment, which offers the Pre-U, said: "The Cambridge Pre-U has been designed for anybody who could achieve a pass at A-level but it does offer stretch and challenge at the top."
A Department for Children, Schools and Families spokesman said: "Time and again A-levels have been shown to be robust and rigorous, backed by universities, and supported by the public. New extended projects, more open-ended questions and the new A* (grade) are making sure that the very bright are given the chance to shine."