Baby boxes are being promoted as a safe alternative to standard cots, bassinets and Moses baskets despite a lack of evidence on their safety, experts warn.
The cardboard boxes, which come with items like clothes and blankets and can be used as a bed, are offered to new mums in Scotland and parts of England.
The Royal College of Midwives wants the scheme rolled out across the UK, saying it offers a "more equal start to life".
But experts have raised concerns about how safe the boxes are to sleep in.
Writing in a letter to the BMJ, Prof Peter Blair and colleagues say that, as the evidence stands, the boxes should only be used as a temporary bed if nothing else is available.
The boxes, which come with a mattress, have routinely been given to every expectant mother in Finland since the 1930s.
Scotland started offering the free boxes to new parents last year, while some NHS Trusts in England have had pilot and full schemes in place for up to two years. The boxes are not offered by health bodies in Wales or Northern Ireland.
The Royal College of Midwives says the boxes can reduce the likelihood of babies sleeping unsafely, either on a surface like a sofa or with parents who smoke, drink alcohol, or take drugs, and would particularly benefit those born into deprived environments.
However, Prof Blair and colleagues, including representatives of the cot death charity Lullaby Trust, said there was a lack of evidence into how safe the boxes are for sleeping in.
Compared with cots, bassinets and Moses baskets, it is harder for parents to easily see their babies when they are sleeping in them, they warned.
And some boxes, particularly those that are not laminated, may potentially be flammable or leave babies more vulnerable to pets and young siblings if they are kept on the floor.
They are also too small for most babies older than three months and there is no evidence on how durable they are, particularly if they become wet or dirty.
The experts also argue that there is no evidence the boxes have helped Finland achieve some of the lowest rates of cot death (sudden infant death syndrome or SIDS) in the world, as has sometimes been claimed.
SIDS rates are equally low in neighbouring countries, such as Sweden and Denmark, where boxes are not provided, they say.
Prof Blair and colleagues say there should be more high-quality studies "to better understand how families use the cardboard baby box, and its safety implications".
'Best start in life'
Responding to the letter, the Royal College of Midwives pointed out that its position statement on baby boxes had acknowledged that there was limited evidence around baby boxes reducing SIDS.
It said that baby boxes need to be safe, of high quality and the box and mattress should meet at least the minimum UK safety standards.
The Royal College of Paediatrics and Child Health said: "Whilst the evidence around the effectiveness of baby boxes is still being debated, what we do know is that they provide all parents with the essentials needed to give their newborn baby the best start in life."
How to reduce the risk of SIDS
- Place your baby on their back to sleep, in a cot in the same room as you, for the first six months
- Don't smoke during pregnancy or breastfeeding, and don't let anyone smoke in the same room as your baby
- Don't share a bed with your baby if you have been drinking alcohol, if you take drugs or you're a smoker
- Never sleep with your baby on a sofa or an armchair
- Don't let your baby get too hot or cold
- Keep your baby's head uncovered. Their blanket should be tucked in no higher than their shoulders
- Place your baby in the "feet to foot" position, with their feet at the end of the cot or Moses basket
Source: NHS website