It will make unpleasant reading for dieters who have been hoping that light exercise was the key to weight loss.

For it seems the most successful slimmers chalk up a marathon a week, according to research.

A study found that those who exercised for 60 to 90 minutes a day - walking the equivalent of a marathon each week - tended to shed at least two stones and kept the weight off for six years.

The data contradicts the popular theory that gentle exercise is enough to stay slim.

The American Association for Advancement of Science's conference on Sunday also heard fresh calls from a British expert for a more urgent global response to the obesity crisis.

Professor Philip James, chairman of the International Obesity Task Force, believes the obesity epidemic is now too serious to be treated as a "lifestyle choice" issue.

"Just as the threat of climate change has begun to be taken seriously when it may be almost too late, the threat of an obesity-related health disaster around the world needs urgent action because, like climate change, its effects are exceptionally difficult to reverse," he said.

The professor wants governments and industry to work together to tackle the causes of obesity - just as they have done to cut carbon emissions.

And he warned the crisis would not be resolved simply by telling people to exercise and eat less.

Calling on world leaders to agree a pact on fighting obesity, Professor James said major changes were needed in food production, advertising and town planning.

Healthy food should be made cheaper, the advertising of junk food to children clearly regulated and towns built not around roads and cars but around walking and public transport.

"We can no longer afford to wait," he said.

"If we fail to act until we have the perfect solution, it will be too late."

In Britain, a quarter of women and a fifth of men are classed as obese, at a cost to the NHS of £1billion a year.