Contrary to the popular perception, smoking does not relieve stress, while quitting does, a new study has found.

British researchers measured anxiety levels in almost 500 smokers before and after they tried to give up and found the claims of benefits associated with nicotine are a myth.

One in five people said they smoked to help them deal with stress.

All took part in an National Health Service (NHS) smoking cessation programme, which involved being given nicotine patches and attending two-monthly appointments.

Six months after signing up for the course, 68 of the 491 participants were still abstaining - and they were less anxious than before.

However, those who tried to give up and failed were more stressed than in the beginning, according to the findings published in the British Journal of Psychiatry.

"The belief that smoking is stress-relieving is pervasive but almost certainly wrong," researchers, from Oxford University and King`s College London, said.

"The reverse is true: smoking probably causes anxiety and smokers deserve to know this and understand how their own experience may be misleading," they said.

Researchers believe the confusion has arisen because one of the withdrawal effects of nicotine is edginess.

Smokers in need of a top-up will repeatedly suffer this feeling throughout the day and find it eases when they have a cigarette, the report said.

While smoking may take the edge off stress related to lack of nicotine, it probably does nothing to ease the pressures of everyday life.

However, researchers are less sure why stress levels rose in those who failed to quit.

They say it may be that their failure - and thoughts of the damage their habit was doing to their health - made them worry more.

This increase in stress levels was particularly high in those with depression and other psychiatric problems and the researchers said doctors should be aware of this.