Figures in the annual report of Her Majesty's Chief Inspector for Schools, Professor Stewart Sutherland, show that at least 1,000 secondary schools are breaking government guidelines on teaching time and that the variation between schools is as much as 20 per cent.
But officials at the Office for Standards in Education admitted that there was no evidence that the number of hours taught reduced standards, though they were investigating a possible connection.
Yesterday's report, the first overview of schools since Professor Sutherland's office was set up, also says that most teaching is satisfactory or of high quality but points to 'a stubborn percentage of unsatisfactory teaching and a small proportion which is downright poor'. This applies to one-third of lessons for seven to 11-year-olds and 20 per cent of secondary teaching.
Professor Sutherland called for an urgent review of the teaching week. 'There is a significant and worrying variation in the amount of taught time. When it adds up to a whole year in five years one begins to realise the effect it might have on a child's development,' he said.
Government guidelines say schools should teach secondary pupils for 24 hours a week, eight to 11-year-olds for 23.5 hours and five to seven-year-olds for 21.
Eamonn O'Kane, deputy general secretary of the National Association of Schoolmasters Union of Women Teachers, said: 'An increased school day would meet tremendous opposition among teachers and would mean a signifcant addition to their workload.'
Professer Sutherland spoke of a year of turbulence and tensions and of the danger of overloading teachers. The report finds that the quality of teaching is satisfactory or better in 70 per cent of primary school lessons and in 80 per cent of secondary ones. Sixth-formers get the best teaching with only 15 per cent of unsatisfactory teaching.
Professor Sutherland said the incidence of poor teaching was often greater in schools in urban areas with high levels of social and economic disadvantage. He has commissioned research into raising standards and into teachers' expectations of pupils. The report says: 'Too often teachers allowed their concern for the pupils' social and economic circumstances to override their responsibility to develop their potential.'
The report notes that bullying affects all schools and that not all deal with it successfully. It says fewer than one-third of primary schools are spending enough time on all national curriculum subjects and religious education. Few primary schools have enough resources for all subjects and nearly one-third of secondaries have shortages, mainly of textbooks, which are lowering standards.
John Patten, the Secretary of State for Education, said: 'Parents must wonder how one school can offer up to a day a week more teaching than another and what effect this has. I certainly do.'
The Government is expected to announce changes to address the problem later this week.