Head teachers have expressed concerns about the initiative, announced four days before this year's GCSE results.

The BBC's Sue Littlemore: "Children as young as 11 will get chance to take GCSEs"

The Secondary Heads Association fears that it could create a two-tier system, where those taking the exams at the age of 16 would be seen as second best.

It is also worried that enouraging young children to take GCSEs could put them under too much pressure, and that those sitting exams early would miss out on studying subjects in depth.

The initiative is seen as the latest attempt by ministers to encourage middle-class parents to keep their children in state schools.

'Bright pupils should be stretched'

It could mean thousands of pupils taking A-levels at 15 or 16, potentially entering university the next year.

Education Secretary, David Blunkett talking to the BBC's Today programme

But Mr Blunkett said there would be no extra pressure placed on pupils, and that the decision on whether children should take exams early would be taken with schools and parents.

Speaking on BBC Radio 4's Today programme on Monday, he said he accepted that some talented pupils would not be emotionally ready to take GCSEs early, and their needs would be taken into account.

But he said it was important for bright children to be "stretched" to prevent them from becoming frustrated.

"It's not the exam that matters - it's being able to take your talent, develop it quickly, allowing you to press on as I've seen in some schools already.

"The important thing here is that they are stretched to a point and in a way that's appropriate to them so that they don't become frustrated and alienated."

The proposals - due to be piloted initially - would only affect about 5% of GCSE pupils, he said, insisting that others would not be made to feel like second class citizens.

GCSE results have improved for the last 11 years

"Let's have a sensible intellectual debate about what's right for the youngsters and how our system - the one that 93% of our children are in - can meet their needs, wherever they are and whatever their background," he said.

Last year just 2,350 children in 1,200 schools sat the exams - which are normally taken at 15 or 16 - early. It many cases this was only in one or two subjects.

John Dunford, of the Secondary Heads Association, said: "I don't want to see a major programme of this happening in education because I think it will give the impression that school education is only about qualifications, it is only about examinations, when it is much more than that."

Children could 'suffer socially'

And David Hart, General Secretary of the National Association of Head Teachers, said: "You need to be cautious about how quickly you do it.

"There is also a danger that if you do it from as early as 11 you lose the opportunity for social contact with the pupil's peer group."

Conservative education spokesman James Clappison said the Government was "missing the point", and that the "rigour" of GCSEs had to be maintained.

"Allowing children to take the examinations earlier - and very much earlier according to these reports - does nothing to uphold the rigour of GCSE examinations," he said.

Last year 46.3% of children got five or more A, B and C grades - a rise on the year before. This year's results are also expected to show an increase.

The plans come a month after the Qualifications and Curriculum Authority recommended that pupils gifted in particular subjects should be allowed to specialise at GCSE level.