A-levels have evolved into a leaving certificate for 18-year-olds

Simon Lebus, head of the qualifications department at Cambridge University, said it was "hard not to be troubled".

He wants an open debate about demands to maintain exam standards while making qualifications more flexible.

Mr Lebus welcomed the creation of a new exams regulator when more alternatives to A-levels were being proposed.

"There is no doubt that confidence in A-levels particularly and also GCSEs has suffered over recent years," he said.

"It is difficult to be precise about exactly what has happened and the whole issue of standards over time is notoriously complex and technically challenging."

Debate over standards

Research from academics and warnings from a government maths adviser of falling standards made it "hard not to be troubled", he said.

A-levels were introduced to "select" applicants for university but have evolved to become more flexible leaving certificates for 18-year-olds.

Government exam reforms could add to the confusion over standards, he said.

From next year, ministers are introducing 17 new diplomas, which could eventually replace A-levels and GCSEs as the "qualification of choice".

There will also be wider use of the International Baccalaureate.

"This also introduces significant scope for confusion as to how the qualifications compare."