In the past year, the three main exam boards - in fierce competition with each other for candidates to follow their GCSE and A-level syllabuses - have developed close commercial relationships with some educational publishers.
They sometimes enter exclusive deals with one publisher, saying their book is approved or "the official textbook". Others endorse the books of several publishers for different subjects. And in some cases, boards charge a "royalty" fee per book sold.
Neither the publishers nor the boards who spoke to The Independent on Sunday would reveal the amounts involved, but informed sources say royalties can range from five to eight per cent of the value of the book. For example, if a popular textbook retailing at around pounds 10 a copy sold as many as 40,000 and the exam board received a five per cent royalty, it would net pounds 20,000 per title.
Once a book carries the exam board logo saying it is the "official" or "approved" textbook, teachers are pressured to buy it rather than other books, depriving pupils of a diversity of choice, critics claim.
The Government's Qualifications and Curriculum Authority is investigating whether the standard of the books which exam boards are endorsing is sufficiently high and whether pupils are missing out on a broader range of reading material.
A spokesman said: "We have the remit to monitor standards and we are talking to ministers about what action to take."
Other sources say the authority plans to issue a new code of practice for boards with a clause banning certain commercial arrangements with publishers. Publishers which do not enter such deals are threatening legal action on the grounds they are "anti-competitive'' and breach European Community law and Office of Fair Trading rules.
Writers, too, are speaking out. Jamie Sale, Society of Authors chairman, said: "A cartel is being created here that undermines fair competition, which restricts public access to the best books and which raises questions about ethical standards of the quasi-public exam boards."
The boards involved are Edexel, OCR (from the merged Oxford, Cambridge and RSA boards) and AQA, (which includes the merged Northern and Associated boards.
In an article in the journal Education in Chemistry, Professor Edgar Jenkins, director of the Centre for Studies in Science and Mathematics Education at the University of Leeds, writes: "The notion of an `official' textbook is not in the long-term interests of pupils, publishers or exam boards."
For example, Heinemann offered teachers "the official textbooks" for London GCSE mathematics - "the fastest- growing syllabus". The Heinemann Modular Mathematics books were promoted as "the official texts for the most popular A-level syllabus". "Applying the adjective `official' to a textbook not only makes clear that it has been approved by an examination board but also implies that it is to be preferred to other `unofficial' text books irrespective of their quality," Professor Jenkins added.
Kay Symons, managing director of Heinemann's secondary division, said teachers were "excited" by textbooks that matched syllabuses.
Publishers which disapprove of such deals, including Oxford University Press and Pearsons, are threatening legal action.