A top exams adviser warns today that the A-level and GCSE testing system is "diseased" and "almost corrupt".

In a new book, Mick Waters, the former director of curriculum at the Qualifications and Curriculum Authority, claims that he witnessed senior exam board officials trying to persuade headteachers that their exams were the easiest in an attempt to win business over their rivals.

He accuses them of contributing to the dumbing down of the examination system – a charge often levied by education traditionalists but strenuously denied in government circles.

Mr Waters says: "I used to think that all this criticism of exams that they were being dumbed down was unfair... [but] we've got a set of awarding bodies that are in a market place. I've seen people from awarding bodies talk to headteachers implying that their examinations are easier."

There are three main exam boards vying for candidates – Oxford, Cambridge and Royal Society of Arts (OCR), Edexcel and the Assessment and Qualifications Alliance (AQA).

The new book – Reinventing Schools, Reforming Teaching – has been written by John Bangs, former head of education at the National Union of Teachers, and Professors John MacBeath and Maurice Galton from Cambridge University. It is the first comprehensive analysis of education policy under New Labour.

Mr Waters also accuses the exam boards of "insider dealing", claiming that head examiners wrote textbooks giving pupils tips for answering questions which they would later mark. He says that Ofqual, the body set up under Labour to oversee exam standards, lacks the courage to tackle the issue.

"I don't think they've got the nerve, and that's where it's so similar to the economic problems where the regulator did nothing new, because the big names went to the Government and moaned. They [Ofqual] should immediately look up whether the chief examiner should be allowed to write the text[book] with regards to pupils' questions. That's insider dealing. You shouldn't be allowed to do that."

Mr Waters told The Independent: "Their exams are supported by texts often produced by their own examiners... there must be a conflict of interest somewhere. Surely it needs looking at."

Last night Ofqual said it would investigate allegations of "insider dealing" if it was supplied with evidence.

Mr Bangs added: "I personally think there should just be a single examination board." He also called for the creation of an Education Standards Authority, which unlike Ofqual would have all its members appointed independently – an idea floated by the Liberal Democrats during the election.

Mr Waters' comments won support from headteachers. Richard Cairns, of the fee-paying Brighton College, said: "I raised this issue with David Willetts [the Universities minister] but the Conservatives seem to favour a marketplace in examinations. The problem is there are several exam boards and they do proffer advice on their own syllabuses. There are also some that are regarded as easier than others and schools that are quite candidly thinking about league tables – they're going to do the easier syllabus."

Exam boards were swift to deny Mr Waters' allegations. A spokeswoman for OCR said: "OCR has never said that its exams are easier. It competes on quality of service, training and support for teachers and educational integrity."

It added that its examiners were banned from using OCR's name for commercial purposes, but added that the board "has no legal right to prohibit them for undertaking work for other organisations as they are not employees". A spokeswoman for Edexcel said: "Edexcel do not agree with Mick Waters." AQA, however, said it wanted a stronger regulator and had a strict code which stated examiners must not make use of its name for commercial purposes.

The book also reveals how Michael Howard, when he was Leader of the Opposition, persuaded Tony Blair to drop his planned reform of A-levels in 2005. An inquiry by Sir Mike Tomlinson, the former chief inspector of schools, was about to recommend that the existing system should be replaced by a diploma covering vocational qualifications as well.