Almost 250 babies died from sudden Infant death syndrome (SIDS) in 2011.
Data from the Office for National Statistics (ONS) showed 244 babies died in an unexplained way in England and Wales, down 17 on the previous year.
In 2004, there were 317 unexplained deaths.
Most (80%) babies die before their first birthday and SIDS is more common in boys than girls (64% of deaths in 2011).
Babies are much more likely to die when parents are unmarried and where only the mother registers the death (0.91 per 1,000 live births compared with 0.19 for babies born inside marriage).
The Lullaby Trust charity's chief executive, Francine Bates, said: "We are extremely disappointed to see such a small, statistically insignificant reduction in the number of SIDS deaths. It's just not good enough.
"We continue to compare poorly with other countries in Europe who have managed to reduce their deaths more significantly. Five babies still die suddenly and unexpectedly every week in the UK. In 2010, the Government scrapped the universal sudden infant death leaflet given to every new parent, which meant we had to replace it with our own resources."
Ms Bates said public health authorities needed to do more to encourage safer sleeping.
Broadcaster Anne Diamond led a campaign to encourage parents to put their babies to sleep on their backs following the death of her son Sebastian in 1991.
Other factors shown to help reduce the risk of cot death include breastfeeding, not smoking, and ensuring covers cannot be pulled over the baby's head.