The Office for Standards in Education announced that inspectors would start writing confidential reports to heads from next term. Details will be sent out this week.
David Hart, NAHT general secretary, who is believed to have proposed the idea to OFSTED, said: "I have no doubt that all heads will deal with this information in a thoroughly professional manner. There is absolutely no justification for suppressing such information."
But the Secondary Heads Association spokesman said: "I can't think of a single head who likes it. It will make decent teachers nervous during inspections and their lessons will look worse."
A confidential report will be triggered when a teacher is graded 1 or 2 (excellent or very good) or 6 or 7 (poor or very poor). The requirement OFSTED circulated for consultation last December was that inspectors should only have to report teachers graded 1 or 7 The judgment can be made after observation of just two lessons. The inspector must warn any teacher judged to be poor that a report to the head is imminent, and provide the teacher with a copy. The teacher can then submit mitigating evidence. The inspector's summary report will not name individuals judged to be failing.
Chris Woodhead, the chief inspector, said it was "the logical next step in raising standards".
Nigel de Gruchy, general secretary of the National Association of Schoolmasters Union of Women Teachers, said: "Any head who did not know already that a teacher was good or bad should not be doing the job."