Parents were warned today that sharing a bed with an infant less than eight weeks old increased the risk of cot death.
Previously it had assumed that under normal circumstances sleeping with your baby was safe and even beneficial.
Only when a mother smoked or had been drinking or taking drugs was the practise thought to be dangerous.
But new research published in The Lancet medical journal says all bedsharing could be unsafe for very young infants in the first eight weeks of life.
The European study, which looked at 745 cot-death cases, confirmed a range of other risk factors that were already known.
They included babies being placed face down, having their heads covered in bedding, and sleeping under a duvet. Smoking in the home was also linked to cot death.
Six out of 10 of all cot death cases in Europe could probably be attributed to lying babies on their front or side, said the researchers.