England's exam system is "diseased" and "almost corrupt", says a former government adviser in a book on Labour's education policy.

A former director at the Qualifications and Curriculum Authority, Mick Waters, says exam boards are conniving in the dumbing-down of school exams.

He also claims the boards profit from textbooks that help teachers with the answers to their own exam questions.

Exam regulator Ofqual said it would investigate claims of impropriety.

The comments come in an interview for a book by the outgoing head of education at the National Union of Teachers, John Bangs, and others.

In Reinventing Schools, Reforming Teaching, Mr Waters says that before working for the QCA he thought all the criticism about exams being "dumbed-down" was "unfair".

"You know, the old argument, more people passed than ever before. Since I've been there, I think the system is diseased, almost corrupt," he says.

This is because exam boards are vying for business in a marketplace, he adds.

He also says he has heard people from exam boards talking to head teachers and trying to sell their qualifications by implying their examinations are easier than those of other boards.

They offer "textbooks to help you through it," he adds.