The number of unexplained deaths of infants has fallen to almost the lowest figure in a decade, it was revealed today.

A total of 221 children aged under one died in England and Wales in 2012. Of these, 158 were classed as sudden infant death while 63 deaths could not be ascertained, the Office for National Statistics reported.

The total is down on 247 deaths the previous year and continues the annual year-on-year fall seen since 2005, when there were 325 deaths.

London had the second lowest infant mortality rate, after the North West, with 0.25 deaths per 1,000 live births. Most infant deaths occurred between 28 days and one year, with boys twice as likely to die than girls.

Factors known to influence a child’s risk of death include the baby’s head being covered during sleep and the danger of overheating from central heating or extra clothing and blankets. Babies that are not breastfed and those exposed to tobacco smoke are also more at risk. New guidance from the NHS is due to be published in December on the risks attached to parents “co-sleeping” with their baby.

Unusually in 2012, the same proportion of unexplained deaths occurred in the summer and winter. Winter is normally a higher risk but the 2011/12 winter was the warmest for five years.

Death rates in 2012 were three times higher for low birthweight babies, while the risk to babies born to mothers under 20 was almost three times higher than average.