In the world's first ever such study, doctors followed 4,000 people over seven years.

They found that over that time, smokers gained as much weight as non-smokers in the same age range.

However, if the smokers gave up, they put on extra weight rapidly.

Smoking status

Dr Robert Klesges led the research at the University of Memphis Prevention Center.

It investigated the relationship between smoking, starting smoking, quitting smoking, and weight change in young adults.

The researchers used information from the Coronary Artery Risk Development in Young Adults (CARDIA) database.

They classified participants into six groups depending on their smoking status - whether they smoked before the study began, whether they started smoking during the seven years of the study, whether they gave up smoking during it and so on.

Participants' provided their current smoking status and body weight after two, five, and seven years.

The researchers found that those who smoked, or began smoking, did not lose weight.

Weight gain

They said the finding goes against the beliefs of both smokers and non-smokers that smoking helps or control limit weight gain.

Smokers "will eventually gain weight"

If smoking can control weight, it is likely to take many years to do so, they said.

They also found that individuals who quit smoking experienced greater weight gain than individuals who continued smoking or never smoked at all - 11.25 kilograms on average.

Weight gain was common among the subjects regardless of smoking status.

Over the seven years, 54% gained at least five kilograms and 29% gained at least 10 kilograms.

This means the gain caused by giving up smoking was on average 5.4 kilograms.

Widespread belief

Dr Klesges said: "These findings have important public health implications, since the perception that smoking controls body weight is widespread, particularly among youth.

"These results demonstrate that smoking does not help control weight, and only after decades of smoking do we see a difference in body weights of smokers and non-smokers.

"If young people can learn that smoking has no effect on body weight, it is likely that a significant reduction among smoking in youth would be observed."

The anti-smoking pressure group ASH (Action on Smoking and Health) said the study had produced important findings.

Clive Bates, director of ASH, said: "This confirms a suspicion we have had for some time - weight gain is part of smoking withdrawal, but smoking itself does not cause weight loss.

"People have assumed that weight control was one of the upsides to smoking. Now it has to be classified as a downside."

Kate Moss: Fashion icon

He added: "Many teenagers are following fashion icons like Kate Moss and mistakenly think that smoking will help them control their weight.

"In fact, starting to smoke has serious consequences for long-term weight gain."

Weight Watchers recommended that if people wanted to lose weight they should take exercise and eat a well-balanced diet.