Not satisfied with pursuing his own strict fitness regime of four vigorous work-outs a week and the odd game of tennis, Tony Blair has declared war on the bulges of the rest of Britain's population.

The Prime Minister has admitted, in a leaked letter, that the Government's efforts to encourage more people to take up sport have failed and an "interventionist" approach is needed to improve national fitness levels.

Writing to Tessa Jowell, the Secretary of State for Sport, Mr Blair said he wanted to use London's bid for the Olympic Games as a catalyst to counter lethargy among adults and children. Downing Street sees increased sporting activity as vital if Britain is to reduce rising levels of obesity in the young and head off massive future health bills for diabetes and heart disease.

Despite new sports centres and facilities using £1.6bn of lottery money over the past nine years, participation in sport has increased by only 0.3 per cent in the same period.

In the letter, obtained by The Observer, Mr Blair cited Labour's aim of getting 70 per cent of the population physically active by 2020, and added: "We need an ambitious delivery strategy, using the Olympic bid as a catalyst, to develop more innovative and interventionist policies across the public, private and voluntary sectors in health and sport if that target is to be achieved."

The process of heaving Britons off their sofas and into their Lycra shorts will begin in the new year with a £1m publicity campaign promoting walking to work, gardening and housework as ways to get fit.

An inter-departmental Whitehall team set up to provide ideas on improving participation in sport has proposed that rather than doctors writing prescriptions for drugs when treating obesity, they could prescribe a course in the gym. Local authorities could be rewarded for making it easier for children to walk to school, and discussions are also being held on the idea of tax breaks for joining gyms.

In America, companies are increasingly taking measures to get employees moving, and the federal government estimates the cost of obesity to be $117bn (£78bn) a year. Capital One, the credit card company, has placed the canteen at its new headquarters at the far end of a string of buildings. and the Union Pacific Railroad is offering some employees weight-loss drugs.

In Britain, a national action plan to make people more active is expected by April.