"I was shocked," said private tutor Som Gill.

The scandal was only uncovered on 13 June, when BBC Two's Newsnight programme discovered that the pure maths paper had been seen by students in north west London.

It came to light when a concerned student handed his private tutor a copy of the paper, saying copies had been sold for �400.

The tutor, Som Gill, who lives in Ruislip, said he was shocked: "It is a troubling thing for students who work very hard, and then we have those other people who cheat."

Edexcel said it had no choice but to let the exam go ahead as planned with the leaked paper.

It has warned that students suspected of cheating will be stripped of the qualification.

Investigations underway

Edexcel spokeswoman Stevie Pattison-Dick said: "Internal compliance investigations have been launched. We are taking this very seriously."

She said the board was attempting to establish the source of the leak, adding: "We are contacting schools to check the seals on the papers."

"It's amazing. I don't know how they got the paper."

The exam board also warned that pupils' results would be compared with predicted grades and any thought to be making use of the leaked papers would be failed.

Retired teacher John Corner criticised the board for having no contingency plan.

"They should have had a back-up paper up at the university centre right now.

"As soon as they realised as they now do that this paper has been divulged and is generally in circulation and obtainable, they should have been getting those back-up papers to all the centres."

Head teachers

But David Hart, the general secretary of the National Association of Head Teachers, said it was too late to swap papers.

"Nothing can be done realistically to stop the examination proceeding today but it is inevitable that some students will gain an unfair advantage and that some students will be treated unfairly as a result of this appalling episode.

"It's not fair. We've been working really hard and they are just cheating."

"Every effort must be made to uncover the perpetrators of this appalling attempt to make money and to cheat on a widespread scale.

Mr Hart said he had never heard of a similar breach of security but called on the examining body to take action to make sure it never happened again.

"This is clear evidence that some students will stop at nothing to get better results and secure university places under the new highly competitive system," he said.

John Dunford, the general secretary of the Secondary Heads Association, said the examining board had done the right thing.

"I support the decision of the examination board to use the existing A-level mathematics paper", he said.

This is clear evidence that some students will stop at nothing to get better results and secure university places under the new highly competitive system.

David Hart, National Association of Head Teachers

"To have changed to a reserve paper at such a late stage would have caused major difficulties."

Nigel de Gruchy, the general secretary of the National Association of Schoolmasters Union of Women Teachers told people not to panic.

"Of course this is a matter for concern, but it is the first such incident and we should not panic. One leak does not make a flood.

"The UK examination system remains probably the most credible and secure in the world," he said.

"Demoralising"

The father of one teenager who sat the exam in London said his son was fed up to hear about the cheating.

He told BBC News Online: "He heard about it last night and was pretty annoyed.

"It must be very demoralising when you sit an exam in these circumstances."

At Holland Park school in west London, pupils who sat the exam were told about the paper scam afterwards.

No pupils there were involved in cheating.

Many of those who took the exam were angry. One student said: "It's not fair. We've been working really hard and they are just cheating."

Another said: "It's amazing. I don't know how on earth they got it and I don't know if it was worth it."

Exam change controversy

The exam paper scam comes amid mounting concern over the growing workload faced by A-level students under new curriculum changes.

Instead of commonly studying three A-levels over two years, students are now often doing four or five AS-levels ("advanced subsidiary") in the first year, then concentrating on A-levels in the second year.

This has also caused scheduling problems for schools, which are having to fit many more exam sittings into the timetable.

And there are fears that it could all be for nothing as universities will continue to focus on the traditional A-level grades.

Last month BBC News Online revealed that trial papers containing actual questions for Key Skills exams had been put on a college lecturer's personal website.

The papers to be sat this month were switched as a result, but the series which was then ongoing went ahead.