Schools could soon be failed by inspectors because their lessons are mostly satisfactory rather than good.
Inspectors have been told to label teaching in some schools unsatisfactory where lessons are judged to have been "generally satisfactory".
All inspectors must follow the guidance under Office for Standards in Education rules. Last year they graded teaching as satisfactory overall in 22 per cent of primary schools and 19 per cent of secondaries.
News of the move will be a shock for schools as Ofsted's 2003 inspection framework had been billed as the start of a "light-touch" regime. It follows controversial comments by David Bell, the chief inspector, over the value of lessons deemed satisfactory.
In his annual report earlier this year, he asked: "Is satisfactory (teaching) good enough given the demands of pupils and the rising expectations of wider society?"
John Bangs, head of education at the National Union of Teachers, has written to Mr Bell voicing concern. "It appears to demonstrate an Alice in Wonderland situation where the meaning of the word satisfactory is the reverse of its "original intention," he says in the letter. Nationally, the number of schools with pejorative Ofsted inspection reports could rise significantly."
John Dunford, general secretary of the Secondary Heads Association, said the guidance "makes a nonsense of the English language".
The guidance is printed in handbooks for inspectors visiting primary, secondary and special schools from the start of this term. It states:
"Teaching that is predominantly and consistently very good with some outstanding lessons may justify an overall judgment of excellence.
"On the other hand, teaching that is generally satisfactory with little that is better merits a judgment of unsatisfactory owing to the lack of aspiration in teaching."
Inspectors are told that an overall unsatisfactory judgment for teaching should, if it is accompanied by another "major weakness", result in a school being placed into special measures. Moreover, the guidance says that a single "major weakness" on its own could, if serious enough, lead to a school being condemned as failing.
Inspectors have already voiced concern to Mr Bangs about people being labelled failures when their performance was satisfactory. They were also worried that there was a push for more critical judgments.
An Ofsted spokeswoman said that judgments about overall teaching quality were not based on lesson observations alone and had to take into account other factors such as the quality of pupils' work.
"A school with a predominance of satisfactory teaching is in the minority," she said. "If, additionally, there is virtually no good or better teaching, then this is well below what is expected of a school. It is reasonable in such rare cases to say that the school's teaching is unsatisfactory overall."