Forty per cent of dieters give in to temptation

The British Dietetic Association said a survey of 4,000 men and women found an obsession with "quick-fix" diets.

It said this led to "yo-yo" dieting, as people tried every new weight-loss plan that came along.

The survey found that over a quarter of people are constantly fighting the bulge.

But over 40% admit they end up giving in to temptation.

Exercise plan

Ten per cent of those who gain weight after dieting put on up to a stone on top of their pre-diet weight.

Women revealed they were more likely to pile the weight back on after a diet, with nearly 40% saying they ended up being heavier than before as opposed to 20% of men.

While a majority of people said they were currently trying to lose weight, virtually all of them said they realised yo-yo dieting was bad for their health.

To try to help people diet more effectively, the British Dietetic Association has launched an internet guide, called bdaweightwise.com to offer practical advice and support, including a body mass index calculator and a home exercise programme.

'Unhappy'

The BDA survey also found that just 18% of dieters ever reach their target weight.

The public does have a love affair with yo-yo dieting Lis Anderson, British Dietetic Association

One in five people manage to stick to their diet for up to a month at a time, while 10% last up to eight weeks.

The survey found that two thirds of men and over three quarters of women are constantly unhappy with their weight.

Almost 20% of men feel like they need to lose between one and one and a half stone (6.35 to 9.52kg). Women's target tends to be smaller, with most wanting to lose between half a stone and a stone (3.17 to 6.35kg).

Lis Anderson, a spokeswoman for the British Dietetic Association, told BBC News Online: "A lot of diets, such as Atkin's, South Beach or the grapefruit diet involve rigorous eating plans.

"People get fed up, and go back to having a few treats. In addition, often they haven't involved exercise in their weight-loss plan, so they do put weight on."

She added: "The public does have a love affair with yo-yo dieting.

"You only have to see what happens when a new diet or exercise book comes out.