The warning will come as several well-known figures kept fat in the headlines. Nigel Lawson, the former Chancellor and embodiment of Eighties excess, is to offer some clues as to how the nation might reverse a slide into obesity in a book about how he downsized dramatically from 17 stone to 12.
The Princess of Wales elsewhere declared war on waifs in the fashion industry and Omega, the watchmakers, abandoned its stance decrying super-thin models to announce it would now advertise in Vogue.
Edwina Currie, a former junior health minister, exposed her former department boss Kenneth Clarke's idea of a diet as "the largest pizza you have ever seen", and a "huge cigar".
A conference of health experts will hear today that one in four Britons will be obese by 2005. The reason is pure laziness, said the Food and Drink Federation, launching an exercise and healthy eating campaign.
The conference, attended by scientists, doctors, government officials and fitness experts at the Royal Society of Medicine in London, was called in an attempt to avert a future peopled by overweight couch potatoes.
Karen Barber, a Food and Drink Federation spokeswoman, said: "Research shows that levels of physical activity have declined dramatically in recent years and this appears to be a major factor in the rise in the number of . . . obese people." The aim of the Join the Activators campaign was to encourage people to take more "moderate, fun exercise" and to eat healthier meals.
Dr Adrianne Hardman, an exercise physiologist at Loughborough University, said: "In England, seven out of 10 men and eight out of 10 women don't take enough exercise."
The initiative has been welcomed by Baroness Cumberlege, a junior health minister. And from September she will be able to recommend The Nigel Lawson Diet Book.
One diet that did not work for Lord Lawson, was the "Whitelaw regime", named after his former colleague Willie Whitelaw. He advised giving up spirits and drinking wine instead. This method "slowed the upward trend" for Mr Lawson but hardly helped him to lose weight. The House of Commons Weightwatchers chapter also proved fruitless.
Even when the former varsity skier fell over while skiing in 1993, and was unable to get up, he kept on eating. The onset of arthritis in his knee, however, started a determined diet that has left him looking half the size of his Cabinet-self.
Last year the ex-food minister Nicholas Soames, known in Westminster as the "Crawley Food Mountain", went on a diet for charity. His progress to date is unfortunately less visible than Nigel Lawson's. "The only thing that worries me is that it is not my nature to be a very thin man," he said. "I do want to enjoy life to the full."