Nicotine - not just in cigarettes but also in patches and chewing gum - could be a major cause of cot death, according to researchers.

It has long been accepted that cot deaths are far more likely in families where one or both parents smoke. According to the Foundation for the Study of Infant Deaths in London, past research shows that babies who die are twice as likely to have been exposed to tobacco smoke than others who do not. In a population where a third of mothers smoked, it has been estimated that cot deaths would be cut by 30-40% if they stopped.

But the latest research, published in the well-respected US journal Proceedings of the National Academy of Sciences, suggests that the causes of these cot deaths could be the nicotine in the cigarette, rather than any combination of toxins from the smoke. The scientists say their findings mean that pregnant women should avoid all nicotine, including gum and patches to help them give up smoking.

"What we have found is that it's the nicotine itself that is dangerous. It doesn't matter if you're using nicotine chewing gum - it's just as bad," said Hugo Lagercrantz, a professor and paediatrician at the Karolinska Institute in Stockholm who carried out the study.

Babies who are considered to have died a cot death, usually at around two to four months, have stopped breathing for an unexplained reason - something the body is normally programmed to avoid.

Receptors in the brain send an alarm to the nervous system when oxygen levels get too low during sleep because of a blocked nose or face buried in a pillow. The message brings you back to consciousness, allowing you to open your mouth or turn over on your back. The researchers believe that nicotine interferes with this warning system.

Prof Lagercrantz carried out a study on mice at the Institut Pasteur in Paris with Pasteur colleague Jean-Pierre Chang-eux and Philippe Evrard from the Robert Debre hospital, also in Paris. They found that genetically modified mice that lack the "wake-up" receptor did not awake although breathing had stopped.

This receptor is also the one that responds to nicotine, so the scientists concluded that in growing foetuses these vital receptors become numb if exposed to nicotine, disturbing the wake-up reflexes and increasing the risk of cot death.

"The researchers proved that the reflex response to the lack of oxygen is diminished in the invalidated (modified) mice, suggesting the involvement of the receptor in this response," the report said.

Prof Lagercrantz is convinced he has found the major cause of cot death. "If women gave up nicotine altogether the problem with cot death could probably be virtually eliminated," he said.

However, the Foundation for the Study of Infant Deaths urges caution. "In some families where a baby dies, neither parent smokes," said a spokeswoman. "There will be a number of different factors working on different babies in different ways. It is all about a vulnerable point of development."

But FSID says that if mothers smoke in pregnancy, their babies are up to 15 times more likely to die a cot death, and they urge both parents to try and give up before trying for a child.