Design and technology, geography and music are the subjects inspectors are least likely to report on when they visit primary schools.
Figures from the Office for Standards in Education show less than half of schools provide enough evidence for inspectors to make judgements on these subjects.
They reflect concerns that the broader primary curriculum is threatened by the dominance of English and maths in the testing and inspection system.
Jeff Holman, assistant secretary of the National Association of Head Teachers, said: "There is an extent to which the inspection process drives what goes on in schools because of its high-stakes nature. It is not just Ofsted, it is also the performance tables. As long as Ofsted and the Government focus on maths and English, that is what schools will focus on."
Ofsted eased its inspection regime in September 2003. It now requires fewer inspections of good schools and puts more emphasis on schools' self-evaluations.
Ofsted said the old system gave a less representative picture of the quality of subject teaching because no subject reports were included for the most effective schools.
Schools are not expected to show inspectors every subject taught in the same depth.
Inspectors must make it clear what evidence was available to them when making judgements but are told: "Do not, however, try to say more than you can. You need to see teaching taking place in order to judge it."
The figures, based on 2,460 reports made since September 2003, show that in almost 6 per cent of schools judgements were not possible on any foundation subjects, a further 24.5 per cent had judgements made on only one or two foundation subjects.
Stiperstones Church of England primary in Shropshire was highly praised by inspectors who visited in September 2003.
They said Mark Klekot, head of the 50-pupil school, had an exceptionally clear vision of how a creative curriculum could raise standards. But they felt unable to make overall judgements on any foundation subjects, apart from mentioning that teaching religious education was good.
Mr Klekot said: "English, maths and science are the core subjects and you can't ignore that. I have no qualms about it. We offer a very broad curriculum and the inspection team had a professional dialogue with me about the whole curriculum."
Earlier this year Ofsted published plans for slimming down the inspection process further, giving schools more frequent but shorter inspections.
Charles Clarke, the Education Secretary, has said a chief adviser on geography will be appointed by the end of the year to help tackle the crisis in primary schools, where it is the weakest subject.