Planned tougher questions for A-level candidates should be compulsory, ministers have been advised.
Few do the existing - optional - extension awards
The Qualifications and Curriculum Authority has looked into operating extended questions as part of an overhaul of qualifications in England.
They are intended to produce A* and A** grades to stretch and differentiate between the best candidates.
The authority says it thinks that if the tougher questions are optional, universities will ignore the results.
The QCA will be piloting both the optional and the compulsory approaches.
But its director of qualifications and skills, Mary Curnock Cook, points out in a letter that the existing Advanced Extension Award (AEA), which was supposed to do the same job of stretching the best students, has largely been ignored.
"Experience tells us that universities find it hard to devise admissions policy based on 'optional extras' as has been demonstrated, for example, by their lack of support for AEAs," she said.
Instead, she proposed that "one or two extra grades would be created, above the current grade A".
She said: "This option may have the potential to promote take-up by more candidates and secure reliable assessment."
People could be taking the new exams by the end of the decade.
The government's proposal of tougher questions - in effect, incorporating the AEA into mainstream papers - was in its response to the Tomlinson Report on 14 to 19 learning.
Sir Mike was also in favour of tougher questions but felt they should be compulsory.
Otherwise, he said in November, the system might disadvantage pupils in schools where staff decided not to prepare for those questions.