The estimate comes from Ken Boston, the chief regulator, of the Qualifications and Curriculum Authority (QCA).

But he said no-one knows how much is actually spent on examinations and tests, and where it comes from.

Consultants have been hired to find out - but he said that the cost to schools and colleges "has soared to clearly unacceptable levels".

Greater workloads

Dr Boston, the QCA's chief executive, repeated his complaint that the system was grossly overloaded.

Without substantial and early reform, the system will continue to operate at unacceptable levels of public risk Ken Boston

In a speech to the conference of the Joint Council for General Qualifications - which represents the exam boards - he said the system had coped this summer because of a "Dunkirk spirit".

Many of the 60,000 exam markers had taken on "significantly greater loads" than in previous years.

But it was clearer than ever that the examination and test system in England was not sustainable, operationally or financially.

"The tolerances however remain far too fine, and can place an examination board within hours of failure to deliver results", he said.

"Without substantial and early reform, the system will continue to operate at unacceptable levels of public risk."

Costs

Reform would cost money but could be achieved more readily if the existing funds were used more efficiently.

As a starting point, it was necessary to know just what the costs were - hence the commissioning of PriceWaterhouseCoopers to find out.

"When one takes into account the costs incurred by schools and colleges in fees, other cash outlays and staff time, together with the operating expenses of examination boards and the regulatory authorities, and direct and indirect government funding of various types, there is evidence to suggest that the total cost might be in excess of �750m per year.

"A huge investment. Yet again, this year, we are likely to see awarding bodies reporting substantial deficits - this is not a profitable business to be in - and the cost to schools and colleges has soared to clearly unacceptable levels."

'No interference'

Dr Boston also rounded on those who suggest there might be any collusion between government, regulator and exam boards to inflate students' results.

"That is not only untrue, but professionally offensive," he said.

There was no pressure from government, either overt or covert, "to contrive a politically convenient result".

Any such conversations "would be regarded as compromising and unethical".

Ministers had been "scrupulous" on this issue.

"Were pressure ever brought to bear by any government on this matter, I assure you that, as a matter of principle, I would be the first to make such impropriety public."