Their report - which shows that there is little connection between the length of the teaching week and exam results - will embarrass the Prime Minister, who suggested in a speech last December that teaching hours should be included in league tables.
John Patten, Secretary of State for Education, ordered a review after a report from the Chief Inspector of Schools showed some pupils were losing a day's teaching each week.
Yesterday's report from the Office for Standards in Education says that teaching hours vary - in primary schools from 20 to 23.8 and in secondary schools 23.3 to 25.8 - though more than 80 per cent of schools have increased their teaching hours since 1989.
The inspectors' conclusions in the interim report on 64 schools are reinforced by an analysis of the 1992 test results and a study of teaching hours and GCSE results. The latter shows 'that the correlation between the time allocated and the standards achieved is at best extremely weak'.
The Government's recommended teaching hours are 21 for 5- to 7-year-olds, 23.5 for 8- to 11-year-olds and 24 for secondary pupils.
The inspectors note that those schools which fail to teach the minimum hours are not covering the national curriculum. In secondary schools, this means that subjects such as history, geography and physical education are neglected. Schools teaching more than the minimum hours were usually providing a better and broader curriculum.
The report points to big differences in the amount of time schools spend on registration and movement. It suggests schools should review the time spent on these and on plays, concerts, sports, photographs and early finishes to term and half-term.
Mr Patten said he had asked the inspectors to conduct a more extensive inquiry into taught lesson time. 'Although taught time is only one amongst a number of important factors contributing to effective education it is rightly a subject that deserves detailed examination.'