Two government departments are in dispute over the time pupils should spend doing PE.
Ministers are considering an increase in the period from two to four hours per pupil per week in a bid to reduce child obesity.
The idea has strong backing within the Department for Culture, Media and Sport but is being resisted by education ministers, who are concerned that it could increase schools' workload.
Charles Clarke, the Education Secretary, denied a report in The Times that the Prime Minister was to announce the increase on December 7.
A DCMS spokesman said: "The Times got ahead of itself. We have been looking at ways to improve what is on offer. This is an option but we genuinely do not know where we are going to end up."
Schools are struggling to meet the Government's target of 75 per cent of pupils taking part in two hours of high-quality PE and school sport inside or outside the curriculum by 2006.
A survey in April found that fewer than two-thirds of pupils in the Government's school sports partnerships do so.
International league tables show that the UK is one of the fattest nations in Europe. One in 10 six-year-olds and one in six 15-year-olds is classed as obese and the rate is rising faster than anywhere else in the developed world.
This week the Central Council of Physical Recreation, the umbrella group for Britain's national governing sports bodies, challenged the Government to double its funding for sport.
It said sport should be given the levels of funding usually associated with the arts.
The CCPR said more funding would make a huge difference to British sport and help London's bid to stage the 2012 Olympics.
Howard Wells, CCPR chairman, believes sport plays a pivotal role in tackling obesity and crime and is central to education.
"When there is such public concern about health, social exclusion and disaffection, yet pride in sporting achievements is so great, sport and recreation have an enormous role to play in the shaping of our nation," he said.