The number of babies dying from sudden infant death syndrome (SIDS) rose in 2013 for the first time in five years, official figures show.
The latest data from the Office for National Statistics (ONS) showed 249 babies died in an unexplained way in England and Wales, the first rise since 2008.
SIDS, also known as cot death, is rare and the risk of a baby dying from it remains low.
Babies between one and four months, premature babies, low birthweight babies and boys are more at risk than others.
Just over half (55%) were boys in 2013.
The most deaths were in February, which was a colder than average month.
The ONS said two risk factors for unexplained infant death are overheating and an unsafe sleeping environment, such as the baby's head being covered. They said these may be more likely to occur during cold weather when extra clothing or blankets are used and central heating is on at night.
In 2012 the figure dipped to a low of 234, but in 2004 there were 317 deaths.
Unexplained infant deaths accounted for one in 10 (9%) of all infant deaths in 2013.
The 2013 figures for February (28 deaths) and April (26) are the third highest for those particular months since records began in 2004.
The North West of England saw by far the biggest rise in the rate of deaths, which more than doubled (114%) year on year. The next biggest jump was in Wales (up 18%). The biggest fall was in south-east England, where the rate decreased by 13%.
Weymouth and Portland in Dorset, Blackpool, and East Lindsey in Lincolnshire have had the three highest rates of unexplained infant deaths in the past 10 years.
In Weymouth the figure was one in every 850 births, in Blackpool one in every 950 births and in East Lindsey one in every 960.
Tower Hamlets in east London has had the lowest rate for the past 10 years: just one death in every 8,300 births. The East Riding of Yorkshire (one in 7,700) and Bexley in south east London (one in 7,100) were the next lowest.