Plans to make A-levels more stretching for the brightest students will lead to a two-tier system, the former chief inspector of schools in England warns.
There is concern about the increasing number of A-level candidates gaining A grades
Sir Mike Tomlinson has raised concerns about the practicality of introducing optional harder questions in exams.
He said the system might disadvantage pupils in schools where staff decided not to prepare for these questions.
Last year, Sir Mike advised the government to phase out A-levels and GCSEs in favour of a diploma.
But his recommendations for 14 to 19 education were rejected by the government.
Instead the Education Secretary, Ruth Kelly, proposed more stretching questions in separate sections at the end of A-level papers, aimed at the top candidates.
Sir Mike's report did propose having tougher questions - but not that these should be optional.
Speaking at a conference in London organised by the National Union of Teachers, Sir Mike questioned whether the government's plans were "technically possible".
"If it's optional, will the option be that of the school or the students?" he said.
Sir Mike Tomlinson's proposals for overall diplomas were rejected
"If the school takes the option not to prepare students for that, we may have very able students in those schools who are not having the opportunity to be challenged, nor to show how much they know about their subject.
"That's the equity issue."
Universities and employers have complained that with so many students scoring A-grades at A-level, it is becoming increasingly difficult to distinguish between the best.
But Sir Mike expressed concern that those bright students who did not take the harder questions would suffer when competing for university places with those from schools which did teach the harder questions.
"You have got to have an assessment system which covers all of those people but which actually gives everyone an opportunity to show their scholarship," he said.
"In that sense 'optional' doesn't make sense."
A spokesman for the Department for Education said the government was "committed to increasing stretch and challenge at A-level".
"The harder question options will be open to all students so that the brightest are able to demonstrate their full ability," the spokesman said.
"The detail of how this will work in practice is being developed by the Qualifications and Curriculum Authority."
The Conservatives said A-levels were not the issue at stake.
Shadow education minister Nick Gibb said: "A-levels are not the problem in our education system.
"We should be focusing on sorting out what is wrong with our education system, not what is working," he said.
"The real agenda for education reform should be one of rigour and autonomy."