Doctors from three medical bodies urged the government yesterday to put together a coherent strategy to fight the obesity epidemic and warned that the public was getting confused by mixed messages on diet and lifestyle.

Carol Black, president of the Royal College of Physicians, said: "What we want to see is coordinated action right from the top, at government level, to have a strategy which will enable us to tackle this problem of obesity." She was speaking at the launch of a report called Storing up Problems - the medical case for a slimmer nation.

The study was produced by a working group from her college, together with the Royal College of Paediatrics and Child Health and the Faculty of Public Health.

It challenges the government to address the issue of obesity, which is damaging health and reducing life expectancy, with a "joined up" strategy which would take in transport, agriculture and food production, health, education and other departments.

John Reid, the health secretary, has held urgent talks with cabinet colleagues in the last few days to prepare for an obesity summit in April.

The meeting will bring together government departments, food companies, nutritionists and consumer groups. "Obesity raises big issues and difficult questions for all of us," he said.

Mr Reid believes in a policy of informed choice helped by better food labelling, as opposed to bans on advertising or sales, as the way forward.

But he is minded to give the food companies a deadline to clean up their act on fatty and salty foods or face direct legislation.

He says he is impressed at the extent to which firms like McDonald's offer healthier options, but believes they need to go further.

The issue will also be addressed by Derek Wanless, who is due to publish a report on public health commissioned by the Treasury near the time of the budget.

The National Audit Office estimates that overweight people cost the NHS at least £500m a year and the economy £2bn. The report said more than half the UK's population - 70% of men and 63% of women - were overweight in 2002.

The incidence of obesity in children is increasing. Among two-to-four-year-old children, it almost doubled from 5% to 9% between 1989 and 1998, and it more than trebled in six-to-15-year-olds from 5% to 16% between 1990 and 2001.

If current trends continue, it is estimated that at least one third of all adults, one fifth of boys and one third of girls will be obese by 2020.

The royal colleges have become concerned by the appearance of what used to be called adult-onset diabetes in children.

Type 2 diabetes is linked to obesity and its complications include blindness and damage to the kidneys.

Peter Kopelman, chairman of the working party, said that the royal colleges were only now producing their joint report, because in the past they had thought the government's initiatives would have more impact than they proved to have.

In 1992, the government set health of the nation targets to cut obesity to the 1980 levels of 8% of women and 6% of men by 2005.

"Nothing happened, despite doctors and physicians saying we need initiatives," Professor Kopelman said. "It is the responsibility of everyone, but it needs government to take the initiative across departments."

Penny Gibson, of the Royal College of Paediatrics and Child Health, said: "We need more coherent and sustained action if it is going to be really effective.

"We have got to provide easy access to good food and alternatives to watching television."

Vending machines in schools selling high-fat and high-sugar snacks had to be removed, she said.

Doctors want to curb advertising of junk food to children on television, but are prepared to engage with the food industry and negotiate. If that does not work, they would like to see legislation.

Prof Kopelman would like to see colour coding for products, with red for high fats or high sugars and green for low. But he said, that if that did not work legislation may be needed.

Above all, the messages going out to the public needed to be clear, he said.

"People are getting very confused by the conflicting messages - what is healthy living and a healthy diet? We recommend a sustained campaign that sends out simple, comprehensive messages about what is meant by enjoying life but being healthy."