Prepare your child for key stage 2 national tests is now in its third print run, with four out of five sold to parents and the rest to schools and advisers. Sales of the booklets, published by Letts Educational, have outstripped those of its more established GCSE revision guides in the same period of time.

Letts has already commissioned a similar guide for national tests at 14 and is considering introducing a version for seven-year-olds. The maths book has been most successful, selling 13,000 copies; English and science have sold 11, 000 and 8,000 respectively. Other organisations say privately they are planning rival schemes to exploit the demand.

But the success has worried those who fear children of better-off parents will be at an unfair advantage and that test results may be used in selection by secondary schools. Last week The TES reported University of London research which revealed that some schools are coaching children through the KS2 tests and recreating the formal exam atmosphere of the 11-plus.

The Letts material openly appeals to parents’ desire for their children to succeed in next term’s tests. “Although the tests are not intended to be a modern 11-plus, they will be of particular significance as children prepare to move from primary to secondary education, especially as the results will be available to the child’s secondary school,” say the booklets. They add: “Good test results, together with a positive report from the primary school, will ensure your child is placed in a higher group (at secondary school).”

As well as sample questions, based on last year’s pilot tests, the Pounds 3.95 booklets explain how national curriculum levels and attainment targets work, provide answers and marking schemes for parents and suggest how answers could be improved.

Arthur De Caux, senior assistant secretary of the National Association of Head Teachers, said the booklets showed that tests were being put before the national curriculum, a reversal of the intended strategy. “This is a typical example of rhetoric and reality being totally turned round. What standards are being raised by this? The standard of children’s ability to take tests. ”

But Margaret Morrissey of the National Confederation of Parent-Teacher Associations said parents could be buying the booklets simply to find out what national curriculum jargon means and what the assessments involve.

Others thought the scheme was a gamble for parents given that this year’s tests will be slightly different from the pilots. Professor Caroline Gipps, dean of the Institute of Education, London, said she was not surprised that sales were so high, but questioned whether the series could be extended to KS1 unless it concentrated on improving simple skills rather than practising questions. “If we are going to coach children for exams it is only fair if they all get the same level of coaching,” she said. “But life isn’t like that. ”

The process of revising for the tests could be a useful habit for younger children to form, according to Tony Millns, spokesman for the School Curriculum and Assessment Authority. “It must be right that parents take an interest in their children’s education and assist them in their work,” said Mr Millns. But he was concerned the booklets could encourage parents to coach for specific questions which would not appear on the real papers.

Jonathan Harris, managing director of Letts Educational, admitted the questions could be of a slightly different sort, but said: “There are only a certain number of ways you can ask questions about that kind of thing.” He added: “Parents are concerned about the tests and there is nothing else to help them. It was a commercial decision to go for KS2 - there was more interest in it than anything else. The arguments put up against them could just as well be used at GCSE and A-level, but nobody minds about that.”