Secret government figures show that between January and May primaries devote 44 per cent of all lessons - or 150 hours in total - to test preparation.
An unpublished survey by the Qualifications and Curriculum Authority reveals that test coaching is stepped up gradually. In January, schools spend an average of nine hours per week on it, building up to 12 hours in April and the first week of May, the survey, in 2003, of 340 schools found.
However, the figures are likely to underestimate the total coaching load in schools because they relate only to test preparation in normal lesson time. One in three primaries told the authority that they offered additional extra-curricular preparation in after-school, lunchtime or holiday "booster classes". On average this coaching lasts 4 hours 20 minutes a week.
And the authority did not ask about test preparation in the autumn term, which is when many schools say they embark upon it.
Separate information from the QCA survey shows how non-tested subjects are cut back: PE, art and music are particularly affected.
The unpublished data showed how nearly 80 per cent of pupils did PE at least twice a week in the early primary years. By Year 6, the figure had dropped to 65 per cent.
Mick Brookes, the general secretary of the National Association of Head Teachers, said: "This bears out what we think is happening in schools: the national tests are having a devastating effect on Year 6.
"The last year in primary should be pupils' very best year. Instead, to spend such a large proportion of the week being trained to regurgitate answers is not a good use of pupils' time.
"However, the NAHT perfectly understands the pressures on schools to raise their results. This adds further evidence to the loud chorus of people saying that the system is wrong."
The figures, the most definitive ever provided on the extent of test coaching, are uncovered in a new book, Education by Numbers: The Tyranny of Testing. They relate to annual monitoring surveys, carried out by the QCA but never published, which have now been released under the Freedom of Information Act.
The QCA figures appear to contradict the Government's position on test preparation. Last December, the Department for Education and Skills said that test coaching should be kept to an "absolute minimum".
The book also reveals the lengths to which the national literacy strategy has advised schools to go to prepare pupils to do well in the tests.
Advice provided in 2003, on how to teach a five-week series of lessons, says children should repeatedly be taken through the mark scheme for the particular exercises they are doing.
The guidance offers transcripts of suggested teaching. In one sequence, having spent a lesson discussing a poem, pupils devote the next two to practising question answering techniques on it.
At Sherwood primary in Preston, the tests do not dominate Year 6. Dave Fann, the headteacher, said that revision did not begin there until just before Easter.
He said: "We try and keep a broad and balanced curriculum as long as possible."
* 'Education by Numbers', by Warwick Mansell (pound;19.99), is published by Politico's next month.