By Adam Brimelow
BBC News, health correspondent
Campaigners want ministers to back a groundbreaking study into childhood obesity which is on the brink of collapse through lack of funding.
The study suggests activity levels are not affected by access to sports
The EarlyBird programme in Plymouth has won acclaim after tracking the development of hundreds of children.
Its findings fundamentally challenge the government's strategy to prevent obesity by raising activity levels.
Ministers say there has never been a commitment to long-term funding, as the research budget is stretched.
Researchers taking part in the study, based at Derriford Hospital, are painstakingly mapping the development of more than 300 children from across the social spectrum.
The aim is to find out more about what lies behind diabetes and obesity.
Every year the children are fitted with accelerometers, which record activity over the course of a week.
They are worn around the waist like pedometers, measuring movement ten times a second.
This gives the researchers a comprehensive picture of just how energetic they are - from the moment they get up until time for bed.
The findings are startling. Take two children on the study - both healthy.
Stephanie Chapman, 11, loves long-distance running at a local track with her friends.
She said: "I think it's really good because you can really come on, you've got to do it.
"You can hear them in the distance and when you come back again they're going well done Stephanie! It's really like Wow! I can do this!"
But Chloe Harris, nine, is much more into singing than sports.
"After a while my legs always begin to hurt, and so do my arms, and things, and also they tire me out and I lose my breath quite a lot."
But the researchers have found that when you look at overall activity levels across the week, Chloe does more than Stephanie.
Professor Terence Wilkin, the programme director, said the amount of exercise children get was genetically set, and had nothing to do with access to sports facilities.
Genes may control how active a child is
"Those children who had little opportunity at school to undertake activity were bouncing around after school whereas those who'd had a lot of opportunity during the course of the school day settled down, and did relatively little," he said.
"The most important thing (was) if you added the in-school activity to the out-of-school activity, they were exactly the same."
That is not the only surprise. Professor Wilkin said children's activity levels had no bearing on their body mass index - their risk of obesity.
"Even looking over a period of years, because we repeat these measures year by year in these children, we have been unable to show any relationship between the physical activity that a child undertakes and his BMI."
If true, these findings cast serious doubts on the government's strategy to halt the increase in childhood obesity by the end of the decade, largely by encouraging physical activity.
Professor Wilkin said it was based on unproven prejudice - that today's children do not exercise as much as previous generations.
His work is widely respected and has produced more than 30 papers in peer-reviewed journals.
But despite local donations and private backing the study is in financial crisis, and his team say ministers have refused to meet them.
Professor Philip James, from the International Obesity Task Force, said it was much easier for the government to concentrate on promoting sport rather than taking on the food industry.
"This is the biggest manufacturing sector in the whole of Europe. It's bigger than the defence industry.
"It has enormous political and strategic power. Government would very much like to focus on physical activity, don't you think?"
Professor James says it is vital that the EarlyBird Study continues its work as the children move on through adolescence.
In a statement the Public Health Minister Caroline Flint said: "Given the immediacy of the target to reduce obesity, the Department's Obesity Programme team is under pressure to identify effective interventions to tackle the problem, and cannot commit its resources to supporting longitudinal studies such as EarlyBird, which lends itself to long-term research funding from a suitable funding body.
"There are huge demands on a finite research and development budget, and most work is funded through competitive tendering exercises."