Many pupils are not receiving the two hours a week of sport that has been promised because the timetabled sessions have been disrupted by exams or time needed to help children change clothes.
The National Obesity Forum, a charity set up to tackle obesity, is calling for schools to ensure by September next year that the two hours consists fully of sport.
In a school sport manifesto, it states: "Until now, many schools have been awarded for demonstrating that they have `timetabled' two hours per week.
"Alas, the ongoing disruption - a lack of hall-time because of other school commitments such as concerts, PTA events, class assemblies, etc - has impacted enormously upon time and delivery."
A survey carried out by the charity last autumn of more than 100 primary school sports coordinators found that nearly half said that some PE time had been lost because of test revision sessions.
Time spent waiting while pupils got changed could also take a significant chunk out of PE time. The younger the pupils, the longer it took. The coordinators said that most Year 6 PE lessons only wasted up to 10 per cent of time changing, while it could take up half the session with some reception groups.
Tam Fry, of the National Obesity Forum's board, said: "The Government keeps saying that 86 per cent of schools are providing two hours of sport, but there is no point in saying that if a school is spending 20 minutes of every hour on pupils getting changed.
"The fact there are more exams also means it is difficult for PE teachers to fight for hall space."
A Department for Children, Schools and Families' spokesperson said: "Schools have a duty to teach the full programmes of study in the PE national curriculum and it is for them to decide how they arrange the timetable.
"The vast majority of any PE lesson is spent on sporting activities and the two hour figure also includes all sport played within school hours, not just PE classes.
"The use of sports halls during exams should not affect the data. The guidance that accompanies the School Sport Survey advises schools that they should collect data on PE and sport from a `typical week' - exam periods are very short and would not represent a `typical week'."