They say they can find no link between the number of hours of teaching and the standard of children's work, although the Secretary of State for Education has ordered league tables on the subject.
Mr Patten said that variations in the school day were 'unacceptable' and called for a full report from the school inspection body, Ofsted, after its preliminary findings showed that the quality of education was far more important than the quantity.
Ofsted's final report, sent to Mr Patten last week, backs up its earlier view. It says that wide variations in the numbers of hours of teaching in schools had little or no effect on achievement or exam results, a finding supported by studies from other countries.
'The amount of taught time spent on a subject is only one of a very large number of factors which may influence the quality of learning. . . . Inspection evidence in general strongly underlines the importance of using time effectively,' it says.
The report adds that it is almost impossible for schools to provide accurate figures on the hours they spend teaching, suggesting that league tables would be worse than useless. Breaks, registration periods and the time spent moving between lessons in different schools are hard to compare, and primary schools cannot give accurate figures for hours spent on English, science or maths because they may all be taught simultaneously in topic work.
Ofsted officials based their findings on reports from almost 200 schools inspected during the current school year, and also carried out a survey of homework in 26 schools.
They found that primary schools taught between 20 and 25 hours per week, and secondary schools between 22.5 and 29 hours. Independent secondary schools, which tend to be selective and to have better exam results than state schools, taught between 23 and 27 hours.
The recommended minimum number of hours teaching for pupils age five to seven is 21 hours, for those age seven to 11 it is 23.5 hours, and for 12-16- year-olds, 24 hours. The inspectors' earlier report found that 45 per cent of schools were teaching fewer than the minimum hours.
In primary schools, the proportion of time spent on English, maths and science varied from 40 per cent to 75 per cent in different schools.
The inspectors said that schools should concentrate on using time well rather than on increasing the number of teaching hours in the week. They did add, however, that some schools with a short teaching week were unable to cover the National Curriculum properly.
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