Ministers were last night under pressure to increase dramatically the amount of sport children play at school to combat the growing epidemic of childhood obesity.

Sports stars, health experts and education leaders gave their backing to a major new Observer campaign to ensure that pupils get more physical exercise.

With obesity rapidly overtaking smoking as Britain's single biggest cause of disease and premature death, experts say the need for children to remain active and energetic has never been greater. Although the Government recommends that every pupil does at least two hours of sport at school per week, barely a third do so.

The Observer today calls on the Government to show it is serious about protecting the health of children by helping schools and teachers to offer more sport. It argues that all children in Britain should be receiving the recommended two hours a week by 2006. The Government's target is a more modest 75 per cent within three years.

Schools should be able to call on outside coaches and fitness instructors to help them run activities; retired sporting figures should be brought back to pass on their skills to a new generation.

To make sport more appealing, the range of activities on offer should be extended to include dance, skateboarding, orienteering and other activities which increase the heart rate.

Education Secretary Charles Clarke welcomed The Observer's focus on school sport and admitted that the Government is keen to see far more being done to revive physical education.

'Sport is central to what schools do. But sport wasn't given the priority it needs in the 1980s, so we have some way to go. We are making progress although we'd all like it to be faster,' he added.

'We're putting the investment in and we're working with schools and organisations like Sport England to turn it round. The signs are encouraging, but we do have to push even harder.'

Clarke, who privately admits the Government needs new thinking on the subject, will study The Observer's proposals.

Chief Medical Officer Sir Liam Donaldson said last night: 'Higher levels of physical activity among children and young people, together with much healthier eating patterns, are the key to averting the potential catastrophic effects of the obesity epidemic which is beginning to emerge.'

Athletics heroine Paula Radcliffe said she welcomed anything which helped to increase young people's access to sport. 'School sport is very, very important. It's vital to get kids active at school and give them as many opportunities as possible to take part in sport. Every young person in Britain should have the same chance to do sport as I had when I was growing up.'

Trevor Sinclair, the England and Manchester City winger, said: 'Any campaign which encourages more sport to be played at school has my full support. Sport helps pupils let off steam outside the classroom and represents an introduction to physical exercise, which is important to maintain through life.

'Anything more the Government can do in this vital area would be welcome.'

Dr Vivienne Nathanson, head of science and ethics at the British Medical Association, said she strongly supported the campaign. 'The BMA would like children to be given the chance to find a sport which they enjoy and are good at so that they pursue this into adult life.'

Professor Philip James, head of the International Obesity Taskforce, who first began to warn about the impending epidemic in Britain, said: 'School sports is completely fundamental to the health of children. We have systematically lost this over the past 15 years, and children are beginning to lose their understanding of how to even play sports.'

The main teaching unions, representing tens of thousands of staff, said the campaign offered a vital opportunity to win children improved sports facilities and greater support to improve fitness levels.

Eamonn O'Kane, leader of the National Association of Schoolmasters Union of Women Teachers, and David Hart, general-secretary of the National Association of Head Teachers, said that confronting the problem should be made a national priority.