Parents have been advised not to sleep in the same bed as their baby in official guidance on how to reduce the risk of cot death.
Current advice: Put your baby to sleep on its back
It had been thought it was safe for them to a share bed if the parents did not smoke, drink or take drugs.
But the Department of Health has modified its advice to parents on the back of research in The Lancet disproving this view.
The study found bed-sharing could be unsafe in the first two months of life.
The DoH leaflet, Reduce the Risk of Cot Death, contains the advice: "The safest place for your baby to sleep is in a cot in your room for the first six months.
How to cut the risk:
Cut out smoking during pregnancy
Fathers should stop smoking at this time too
Do not let anyone smoke in the same room as your baby
Place your baby on its back to sleep
Do not let your baby get too hot
Keep your baby's head uncovered
Place your baby with its feet to the foot
of the cot
Preferably sleep your baby in a cot in your bedroom for the first six
Do not sleep with a baby on a sofa, armchair or settee
If your baby is unwell, seek medical advice promptly
"While it's lovely to have your baby with you for a cuddle or a feed, it's safest to put your baby back in their cot before you go to sleep."
The leaflet, distributed to all maternity units in England, emphasises the risk of sleeping in bed with a baby if you are a smoker, even if you never smoke in bed.
It is estimated that seven babies a week in the UK die from cot death - known technically as sudden infant death syndrome (Sids).
Cot death experts from across Europe collaborated on The Lancet study, which focused on 745 cases.
They found a two-week-old child sharing a bed with non-smoking parents was at two-and-a-half times the risk of cot death they would be if sleeping separately.
At eight weeks they were at one-and-three-quarters the risk.
The study did not look at why sharing a bed with a small baby might increase the risk of cot death - further research is required to pin the reasons down.
Joyce Epstein, director of the Foundation for the Study of Infant Deaths, welcomed the new advice.
She said: "We hope that this new research-based advice, coupled with FSID's own baby safety information, will help parents keep their babies safer than ever before."
Rosie Dodds, of the National Childbirth Trust, voiced concerns that the DoH had been too quick to change its guidance.
She said there was still not a significant body of research to suggest that bed sharing was dangerous, but there was research to show that bed-sharing was linked to higher rates of breastfeeding.
It is widely accepted that breastfeeding provides the best possible source of nutrition for babies.
The DoH leaflet is available by calling 08701 555 455.