A-levels have been getting easier, says the academic recruited by Prime Minister Gordon Brown to advise on raising maths standards.
A new A* grade is coming, because so many now get A grades
Sir Peter Williams, chair of the Advisory Committee on Mathematics Education, says that the exam is less difficult than 20 or 30 years ago.
A-levels have been the "gold standard" of the exam system - but Sir Peter suggests that standards have slipped.
The government says the exam watchdog checks that standards are maintained.
Sir Peter was recently unveiled by the government as the academic who would lead a review into primary maths - a centrepiece of the incoming prime minister's plans to raise school standards in England.
But in an interview with a newspaper, Sir Peter suggested that in "absolute terms" the A-level exam was not as difficult as a generation ago - and that there had been a decline over a long period of time.
Using the example of maths and physics, Sir Peter said that the equations currently used in exams were simpler and that material which had been studied in the first year now had to be pushed back to the second year, to allow students to catch up.
A spokesman for the Royal Society, where the maths advisory committee is based, said that Sir Peter stood by the comments about A-levels.
The Department for Children, Schools and Families defended the quality of the qualification.
"The Qualifications and Curriculum Authority has a regular programme of monitoring and quality assurance and these most recent studies show that the exam standards have been maintained over time," said a DCSF spokeswoman.
"In 2005 the independent committee on exam standards concluded that no international system at school, or any other level, was so tightly or carefully managed as our own."
But there are plans to introduce a higher grade - the A* - for pupils who score 90% or more in the exam.
This is in response to the growing number of students who achieve the top grade - with a quarter of exam entries currently getting A grades.