Co-ordination Group Publications, which has a turnover of around pound;15 million, makes most of its money by selling directly to schools but it also produces most of the best-selling revision guides. The company was set up less than 10 years ago by Richard Parsons, a physics tutor working from home in Broughton-on-Furness.

“Teachers are using our books in the classroom or as homework,” Simon Cook, communications manager at CGP, said. “Our products are not there to replace teachers but to support them.”

Mr Cook said the company began by selling GCSE maths guides directly to schools. Its books did not appear in the shops until 2000.

“We have amusing cartoons and jokes in our books. You can say it’s a bit cheesy or corny, but revision can be a dull process for a lot of students, particularly those who are struggling.”

Peter Stafford, sales and marketing director at Letts Educational, said the revision guide market had expanded by more than 20 per cent in the past four years.

He said that although its guides were not bestsellers individually, the company retained a 30 per cent market share by covering a much wider range of subjects than CGP, and selling more in the top price range.

Mr Stafford predicted that the market in revision guides would continue to expand.

“Hardly a week goes by without attacks on exam achievement, which means more and more revision books coming into the hands of parents, who are keen to improve their children’s results.”

Susan Elkin, chair of the Society of Authors’ educational writers’ group and an English teacher for 37 years, said: “What on earth is the matter with teachers when they need revision guides to teach English literature? Exam results should be the icing on the cake, not the tail that wags the dog.”

David Cowley, professional officer at the Association of Teachers of Mathematics, said revision guides could be very valuable when used by teachers in the right situation. “Students can look at a revision guide all day and still not understand,” he said.