Children as young as seven are to be targeted in a multimillion pound campaign by Cadbury to encourage them to buy chocolate bars in exchange for new school sports equipment.

The scheme, to be launched after Easter with the backing of the Government, has drawn condemnation from health experts as a cynical marketing ploy.

Tony Blair's officials were briefed on the £9 million campaign earlier this year, as was Sports Minister Richard Caborn, who gave Cadbury a glowing endorsement.

Primary and secondary schools across Britain will receive information packs asking them to collect tokens for new equipment such as balloon balls and foam javelins. Athlete Paula Radcliffe and boxer Audley Harrison, who have signed lucrative sponsorship deals with the company, are supporting the Get Active Campaign.

But obesity experts believe the food industry is engaged in a long-term strategy to persuade a sceptical public that it is physical inactivity, rather than diet, that is to blame.

The one in 10 children in the UK categorised as obese may well develop serious health problems such as diabetes, heart disease and cancer. Many overweight children are bullied at school, leading to underachievement and feelings of inadequacy.

NHS clinics are encouraging children to exercise to combat problems associated with a sedentary lifestyle. Doctors believe that diets high in fat, sugar and salt are a major factor.

Cadbury's campaign will be the largest of its kind mounted in the UK. More than 160 million chocolate bars, including Crunchie, Dairy Milk and Flake, are to carry the tokens and will be on the shelves after Easter.

Under the 'sports4school' scheme, a school has to collect a minimum of 750 tokens before it can gain the first bit of equipment. With 2,000 tokens, all the equipment is obtained. Many schools are likely to leap at the chance, as budgets for gyms have been slashed in recent years, and there are fewer playing fields for the traditional sports.

Last year, Walkers Crisps launched its latest Books for Schools scheme and Tesco offers computers to schools, using vouchers. Health experts say there is no code of practice governing the marketing of junk food, despite growing evidence of companies trying to encourage brand awareness in children as young as four.

Dr Ian Campbell, a family doctor and chair of the National Obesity Forum, said: "Chocolate is fine as a treat, but not for daily consumption - it's not a health food. My concern is that the company is making a direct link between eating chocolate bars and exercise."

Kath Dalmeny of the Food Commission said: "It doesn't make sense that you have the Department of Health trying to tackle obesity, when Number 10 is being briefed on a campaign aimed at getting children to eat more chocolate."

Yesterday, a No 10 spokesman said: "We don't provide a running commentary about meetings here. This is an independent campaign by a private-sector organisation."