Research into dozens of infant fatalities identified as the result of sudden infant death syndrome showed that the victims had a brain abnormality that prevents the detection of insufficient oxygen levels in the body. As a result, babies with the condition can be smothered in their bedclothes, especially if they are sleeping on their fronts.

The researchers said yesterday that this was the strongest evidence yet of a common cause for cot death, and that it opened up the possibility of detecting those at risk and treating them.

Three hundred babies died of cot death in Britain last year, a 16 per cent fall on 2004. The year-on-year decrease is credited to greater alertness among parents and better ways of determining causes of death, resulting in fewer cases classified as cot death.

The US team, led by David Paterson, of Boston Children’s Hospital, examined post-mortem samples from the brainstems of thirty-one babies who had suffered a cot death, comparing them with ten babies who had died of other causes. They were following up research suggesting that cot-death babies had an innate difference in the brainstem, the part of the brain responsible for controlling breathing, heart rate, blood pressure, temperature and arousal.

Three studies had found that cot-death babies had a reduced ability to use and recycle serotonin, a chemical best known for regulating mood but which has other roles. Scientists said the new study, which was published in the Journal of the American Medical Association, offered the most convincing confirmation yet of the link.

The babies examined had twice as many brain cells that either manufacture or use serotonin as did those of the babies who died of other causes. But the cells that use serotonin also had significantly fewer binding sites — places on the outside of the cells where serotonin “docks” and acts as a signalling chemical.

The study suggests that the slight abnormalities in the brainstem may impair a baby’s ability to sense high carbon dioxide and low oxygen levels in its body. This would increase the risk that a baby will inhale its own exhaled breath and become deprived of oxygen. Hannah Kinney, the paper’s senior author, said: “These findings provide evidence that sudden infant death syndrome is not a mystery but a disorder that we can investigate and some day may be able to identify and treat.

“A normal baby will wake up, turn over, and start breathing faster when carbon dioxide levels rise.” But in babies who die from sudden infant death syndrome, defects in the serotonin system may impair these reflexes.

Such circumstances are far more likely to arise if a baby is placed face down in the cot. Campaigns to put babies on their backs have had great success, halving the numbers of cot deaths in the past decade.

Strikingly, boys who had died of cot death had significantly fewer serotonin binding sites than girls, a finding that would be consistent with the fact that cot deaths are more common among boys.

The Scottish Cot Death Trust, which part-funded the new research, said: “It looks like a really interesting piece of work and we welcome it as a way of starting to sift out the many possible factors in cot death.”

Marian Willinger, of the US National Institute of Child Health and Human Development, which funded the study, said that the research improved the understanding of the developmental disease process that underlies cot death, and the chances of providing at-risk infants with appropriate interventions.

However, George Haycock, scientific adviser to the Foundation for the Study of Infant Deaths, added that a brain abnormality was unlikely to be the sole cause.

“Much more research is needed in order to understand and, ultimately, to prevent these tragedies,” he said.

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