Smoking during pregnancy is already known to increase the risk of cot death but researchers now believe that nicotine from gums or patches could also pose a risk.

Cot deaths occur when apparently healthy babies die in their sleep when their breathing stops suddenly.

Tests on mice showed nicotine interfered with genetic messages which send an alarm from the brain to the nervous system telling it oxygen levels are too low - because something is stopping a person breathing properly.

When the message system is working, the person wakes up and moves so they can breath properly again.

It is better for a baby to exposed to a small amount of nicotine than a lot of cigarette smoke

Foundation for the Study of Infant Deaths spokeswoman

Nicotine disrupts this ability to regulate breathing during sleep say the researchers, from the Karolinska Institute in Stockholm and the Pasteur Institute in Paris.

In their study, they examined the role of a receptor in the brain which raises the alarm when oxygen levels are too low. It also responds to nicotine.

The researchers suggest that in developing fetuses the receptors become numb if they are repeatedly exposed to nicotine, meaning the "alarm" mechanism does not work properly, and increasing the risk of cot death.

Nicotine injections

The team looked at mice, some of which had been genetically modified so they did not have the key receptor.

How to reduce cot death risk

Place your baby on its back Mothers and fathers should stop smoking in the pregnancy No one should smoke in the baby's room Do not let the baby get too hot Keep the baby's head uncovered

These mice did not wake up when their breathing stopped, which the researchers say demonstrates the receptors role in alerting the body to a lack of oxygen during sleep.

In addition, injections of nicotine injections affected the breathing responses of normal mice when they were sleeping but had no effect on the genetically modified mice.

> Professor Hugo Lagercrantz, a paediatrician at the Karolinska Institute who led the research, told Reuters: "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.

"If women gave up nicotine altogether the problem with cot death could probably be virtually eliminated."

Important to quit

But a spokeswoman for the Foundation for the Study of Infant Deaths told BBC News Online: "Smoking during pregnancy and after the baby is born increases the risk of cot death.

"Nicotine gum and patches are used to help the parents cut down and preferably give up.

"Because the risk of cot death increases with every cigarette smoked it is really important for parents to cut down or quit altogether.

"However, it is better for a baby to exposed to a small amount of nicotine than a lot of cigarette smoke which contains many different toxins."

Clive Bates of Action on Smoking and Health said: "There's always been a great deal of hesitation about using nicotine replacement therapy (NRT) with pregnant women because it's known that the fetus ends up exposed to nicotine and that's not a good thing.

"But the best thing you can do for your health and your baby's health is to quit smoking. If using NRT helps you get to that point, it's worthwhile."

The research is published in the journal Proceedings of the National Academy of Sciences.

Cot deaths affect around one baby in 2,000 in Europe and double that figure in North America.