An annual injection that takes the pleasure out of nicotine could be the key to preventing former smokers falling victim to their old cigarette craving.

British researchers developing the vaccine, which has completed its first round of trials on volunteers, say the jab is unlikely to help smokers quit, but might prevent a relapse several months on.

It could even be possible for parents to vaccinate teenagers against nicotine, although researchers concede that this would raise ethical questions. A vaccine for cocaine has also begun clinical trials with similar results.

The drug works by alerting the body's immune system to the presence of nicotine in the blood. Nicotine molecules are normally too small to trigger an immune reaction and slip through the blood-brain barrier undetected.

The vaccine turns them into a target for antibodies by attaching a marker protein. With the antibody attached, the nicotine molecule is too big to get into the brain and so cannot trigger the smoker's high.