Young people face a "physical and mental illness time bomb" unless they get more chance to play outside, a conference has heard.
Inner-city children need more playspace, it was argued
Professor Lamine Mahdjoubi, of the University of the West of England (UWE), said a lack of outdoor exercise caused heart problems and diabetes.
Councils must create more open spaces, he told a conference in Bristol.
The comments come as a survey suggests British children are among the unhealthiest and unhappiest in Europe.
Prof Mahdjoubi told the UWE-organised conference: "As councils cut their spending on outdoor spaces and society encourages physical idleness, obesity is becoming an epidemic.
"Young people now face heart problems, diabetes and other diseases because of their sedentary lifestyles.
"This puts them at risk of premature death and confronts the NHS with a rocketing bill."
Prof Mahdjoubi added: "This is a subject that affects us all. The cost of obesity will have to be met ultimately by society - it is a time bomb waiting to go off, like climate change."
The conference heard that open spaces were more cost-effective, and of benefit to more people, than indoor fitness centres.
Marie Forsyth, of the parents' group To Play or Not to Play, said: "The streets are unrecognisable from our youth.
"Now, they are full of traffic, and play spaces are desolate and scarred by drug-taking. "
Low EU ranking
According to the Prof Mahdjoubi, investment in urban parks and open spaces dropped from 44% of local authority spending in 1976/77 to 31% by 1998/99.
Professor Dick Jackson advises President Bush on the impact of the built environment on health and warned delegates that the UK was catching up with American levels of childhood obesity.
Mental illness could be directly attributed to poorly designed neighbourhoods, he added.
Meanwhile, a York University study has ranked Britain's children 21st out of 25 EU nations in terms of "wellbeing".
Researchers found 16% of children were overweight and only 27% ate fruit every day.
The Department for Culture, Media and Sport is responsible for "play policy" across government.
A spokesman said: "Government recognises that play is of fundamental importance to children and young people¿s health, well-being and learning and, for this reason, there are many initiatives to help promote outside activities for children in a safe and welcoming environment.
"The protection of existing children's play spaces, which are an important part of local communities, is also the focus of a national planning policy framework lead by the Department for Communities and Local Government's (DCLG)."