Inspectors, subject specialists and researchers say PE, music and languages in primary schools are increasingly taught by outsiders during teachers'
They believe that while such staff bring benefits they do not always have the skills to teach primary children.
Inspectors said more and more heads were making indiscriminate use of sports coaches to cover the half-day a week schools have to give teachers to plan, prepare and assess children's work.
"This is threatening high quality provision in these schools," said Ofsted in a recent report on primary PE.
All teachers have been entitled to half a day a week out of the classroom for planning, preparation and assessment (PPA) since September. But primary heads have complained that they do not have the cash to provide cover during PPA time.
A TES survey last term found that 35 per cent of primaries use PE specialists, including sports coaches, to provide cover, 30 per cent use music specialists, and 7 per cent use art specialists.
John Matthews, chief executive of the Physical Education Association of the UK, said: "Some headteachers are forced down this route because of inadequate funding. We are comfortable with coaches supporting schools in out-of-school activities but PE needs to be taught by a suitably qualified teacher rather than a coach."
At Hareclive primary, Bristol, Brian Hall, the headteacher, uses teachers and coaches for PE. The school opened a new sports hall last year with a climbing wall and offers sailing, archery and gymnastics as well as football, swimming and dance.
Mr Hall said: "We are forcing people in primary schools to be experts at 12 subjects and it's just not feasible. If you have a coach with a fantastic ability, I see no problem with making the most of that. What is important is for teachers to take some PE lessons, because the minute you give over a complete subject, the message to the children can be the subject is not important because the teachers don't do it."
This week the National Association of Music Educators advised members that music classes provided in PPA time must relate to the national curriculum and be discussed with pupils' main teacher.
Sarah Hennessy, chair of the association, said: "We are concerned that all the attempts to strengthen music in the curriculum by giving teachers support, advice and opportunities for professional development could be whittled away by this sort of thing (letting teachers opt out of music)."
Foreign languages are not yet part of the primary curriculum, but schools are expected to offer classes in the subject by 2010.
An evaluation of schools in 19 authorities which have been trialling the introduction of languages has also recommended the effect of PPA be monitored.
The report, by a team from Warwick university, said that employing outside language experts could damage long-term plans to enable primary teachers to teach languages.