More than half of all cot deaths take place when the baby is sharing a bed or sofa with a Parent or parents, and may be linked to the mother or father having been drinking or taking drugs, a new study shows.

The issue of co-sleeping has been controversial. Many mothers want to take their baby to bed with them to feed them easily in the night. But the study suggests there is a real Risk when a parent falls asleep with the baby, particularly on a sofa, while under the influence of alcohol or drugs.

The new paper, published by the BMJ, was written by researchers at the universities of Bristol and Warwick, who studied all unexpected infant deaths from birth to two years in the south-west of England from January 2003 to December 2006.

The team investigated the circumstances of 80 deaths and interviewed the parents shortly after each one, collecting information about drink and drug use.

More than half the deaths (54%) occurred when one or both parents were sleeping in a bed or on a sofa with the baby. In similar groups used for comparison, 20% of parents slept with their infants.

But much of the risk, say the authors, can be explained by the parent having been drinking or taking drugs, which happened in 31% of cases in which babies died, compared with 3% of the control group. The biggest risk appeared to be falling asleep after drinking or drug-taking on a sofa, which happened in 17% of cases but hardly at all (1%) in the comparison groups.

A fifth of the infants who died were found with a pillow and a quarter were swaddled, which the authors suggest may also be risk factors. The risks remained the same regardless of socioeconomic circumstances. The authors say advice to parents now must be considered carefully.