One in seven inspection reports on primary schools contains judgements on subjects which inspectors have not seen being taught, the Office for Standards in Education has admitted in its newsletter.
New guidance has now been issued to inspectors on subject judgements. It says that if inspectors have not seen teaching in a particular subject, it is unlikely there is enough evidence to support an overall judgement.
Inspectors are not expected to see lessons in every subject and the evidence they collect for their reports can come from examining pupils'
work, speaking to pupils or looking at assessments and records.
But the Ofsted handbook states that inspectors must give greatest weight to lesson observations when they make judgements about teaching.
Chris Davis, of the National Primary Headteachers' Association, said: "It is exceedingly difficult for inspectors. On one hand most schools approve of shorter inspections but then we have to accept the fact that inspection teams won't be able to see everything in depth.
"Several of my subject co-ordinators were upset that their subject was dealt with by a cursory sentence saying it was not possible to pass judgement, but there is no going back to the days of long inspections.
"Making a judgement on one lesson alone is probably worse than a proper look at the records. Anyone can put on a show for one lesson. To make a sound judgement inspectors need to see children's work over a period of time and have a proper interview with a subject co-ordinator."
The National Association of Schoolmasters Union of Women Teachers also said it did not want this to lead to inspectors visiting more lessons.
A spokesman said: "There is no need to have a rash of inspectors in a school inspecting every single subject and potentially every single lesson in some small schools.
"Schools should already have in place adequate systems for monitoring and accountability which require teachers to demonstrate not only the adequacy of their preparation but the progress of pupils.
"If Ofsted was to focus on those arrangements for assuring high-quality teaching and learning, then it seems a better way to focus resources and time."
Figures from Ofsted released last month revealed that inspectors made judgements on geography or design and technology in fewer than half of schools.
Music was judged in only half of schools visited since the new inspection framework began last September.
English, maths, science and information and communications technology lessons were seen in more than 97 per cent of schools.