Half of the subjects timetabled are never inspected at thousands of primary schools, official figures suggest.
Results of 2,460 reports made since the new inspection framework began last September reveal that inspectors were able to make judgments on English, maths, science and information and communications technology in more than 97 per cent of schools.
But there was a wide variation in how often other subjects were assessed.
The subjects seen least often were design and technology, and geography.
Fewer than half of schools had enough lessons or work available to allow inspectors to make a judgement about these.
A judgment in music was only possible in half of Office for Standards in Education visits.
Of the remaining four "foundation" subjects, only religious education was observed in more than two-thirds of schools.
Unions warned when the light-touch regime was introduced that shorter inspections could mean foundation subjects were neglected.
The figures come as the Government launches its music manifesto, which aims to improve the current "patchy and diffuse" service for children. It repeats the Government's pledge to give every primary child in England the chance to learn an instrument.
A spokeswoman for Ofsted said: "Inspections are shorter than they were and this makes it impossible to evaluate every subject. Although inspectors only make an overall judgment about the effectiveness of a subject where they have enough evidence, all the evidence gathered from seeing a very few lessons in foundation subjects will contribute to the team's view of the school as a whole."
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