A study published in the Lancet medical journal showed that smoking generates a skin enzyme that breaks down collagen, the connective tissue which keeps people fresh-faced.

Sunlight is thought to have the same effect, especially on the face, which is the part of the body most exposed to ultraviolet rays. The combination of smoking and sunlight produces wrinkles, sagging and pallid skin, scientists believe.

Smoking has long been associated with premature ageing. But the reason for the ageing effect of tobacco has not been clearly understood.

Scientists at the St John's Institute of Dermatology at St Thomas's Hospital, London, found evidence that smoking activates genes responsible for an enzyme called matrix metalloproteinase (MMP-1) which degrades collagen.

In a study of 14 smokers and 19 non-smokers, the smokers had significantly more genetic material associated with MMP-1.

Professor Antony Young, who led the research reported in the Lancet, said: "It was a highly significant result and very unlikely to be due to chance.

"Smoking exerts such a noticeable effect on the skin that it's often possible to detect whether or not a person is a smoker simply by looking at his or her face. Smokers have more wrinkles and their skin tends to have a greyish pallor compared to non-smokers."

He said the discovery was made by accident as his team was investigating the effects of sunlight rather than smoking. Volunteers had patches of buttock skin irradiated with ultraviolet light to see if this induced the formation of MMP-1. Previous laboratory studies had indicated a link between the enzyme and sunlight.

The scientists measured levels of messenger RNA (mRNA), a genetic material that helps decipher genetic codes to create proteins. They found that some volunteers not exposed to ultraviolet radiation nevertheless had unusual amounts of MMP-1 mRNA in their skin. Checks showed that these participants were smokers.

The 14 individuals, 11 men and three women, had an average age of about 30 and had smoked 10 to 20 cigarettes a day for between three and 25 years.

Young said he suspected that the face received a "double whammy" of MMP-1 when a person smoked, both from the action of sunlight and tobacco.

Normally the enzyme is used during growth to assist tissue remodelling, breaking down certain structures so they can be rebuilt with new ones. In the body its effect is regulated by an inhibitor substance called TIMP.

But the study suggested that in smokers levels of MMP-1 went up without a balancing TIMP increase. Professor Young said he had no idea why tobacco affected MMP-1. "Tobacco smoke contains thousands of chemicals," he added. "It's very hard to know which ones are active."

He hoped to see the study followed up with a more detailed and larger investigation.

Amanda Sandford, research manager for the anti-smoking group Action on Smoking and Health (ASH), said: "It's ironic that teenagers often start smoking in the hope of appearing more mature, but it probably never occurs to them that by middle age they really will start to look older than their true age."

"For smokers, middle age starts in their early 30s as the tell-tale wrinkles around the mouth and eyes begin to appear," she added. "Young female smokers are likely to be wasting money on anti-ageing creams if they continue to smoke. The best beauty treatment by far is to quit smoking," Sandford said.