And he committed a future Tory administration to "strengthening" A levels by introducing a new A* grade for high performing students.

Mr Hague told sixth form students that the new grade should be introduced to reward particularly high scores - along the lines of the A* grade already awarded for outstanding performance in GCSE exams.

He said his party was concerned that the government's review of qualifications for 16 to 19-year-olds would lead to a weakening of the "gold standard" of A levels.

William Hague: "Instead of diluting A levels, we need to strengthen them"

Ministers have already approved a series of changes to qualifications for 16-to-19-year-olds in an attempt to encourage a move away from the traditional "narrow" focus on three A levels.

They include more opportunities to "mix and match" academic and work-related subjects, and the chance for A level students to study up to five subjects.

"We believe that A levels should be maintained as the gold standard in education, and should be protected and strengthened," Mr Hague said in a speech at The Folkestone School for Girls.

"A practical way of strengthening A levels would be to give more encouragement to young people to aim for excellence. We would achieve this by introducing a new A* grade at A level.

"This new grade would ensure rigorous academic standards and spur young people on to the highest level of academic achievement."

But Mr Hague's proposals were attacked by the Secondary Heads Association. Its General Secretary, John Dunford, said the introduction of A* grades would make A levels even more "elitist".

David Hart: "Lack of government enthusiasm"

"To introduce A* grades will devalue the A grade and cause more stress amongst the most academically able, as it has already done at GCSE level."

And another headteachers' union says the government's plans to reform 16-plus qualifications and encourage more pupils to stay on at school do not go far enough.

The General Secretary of the National Association of Head Teachers (NAHT), David Hart, said the reforms were half-hearted.

"The government has indicated that it wishes to promote broader programmes of study at 16-plus, but the NAHT remains unconvinced that the government intends to 'go for breadth' in any meaningful way," he told a conference of secondary headteachers and deputy headteachers.

The planned reforms were voluntary and little effort had been made to persuade universities or employers of their worth, said Mr Hart.

"The real reason for government dithering is the fear of allegations that the reforms mean 'the gold standard of A level' is at risk.

"The danger is that lack of government enthusiasm, combined with reliance upon a voluntary system, will lead to nothing like the broadening necessary for 16-plus students.

"This would be a tragedy, bearing in mind that the current system serves only a minority and provides little, if anything, by way of relevant programmes of study for the majority."

The government's reforms of 16-plus qualifications are due to be introduced from September next year.