Road casualties in 20mph zones increased by almost a quarter in 2011, according to official figures.
The number of people killed or injured on roads in built up areas with a speed limit of 20mph totalled 2,262 in 2011, up by 24% from 2010.
Meanwhile casualties on 30mph roads were down by 1% from 2010, recorded at 125,494 in 2011, according to data from the Department of Transport.
Councils have been given powers to set 20mph zones in a bid to improve road safety.
Local transport minister Norman Baker said local authorities were best placed to make the decision on whether or not to impose 20mph limits in certain areas.
He said: "British Medical Journal research has shown a reduction in casualties and collision of around 40%, a reduction in children killed or seriously injured of 50% and reduction in casualties among cyclists by 17%.
"That is why we believe 20mph speed limits are useful in certain residential areas and support their introduction where it can be shown that they benefit road safety and quality of life."
He added: "It's vital that speed limits are suitable for local conditions and councils are best placed to determine what these limits are, based on local knowledge and the views of the community.
"Authorities up and down the country have been concluding that 20mph limits are indeed beneficial to their local areas."
Kevin Clinton, head of road safety at the Royal Society for the Prevention of Accidents, said: "Twenty mph roads, especially in 20mph zones that have traffic calming measures to make them 'self-enforcing', are very effective at protecting people, especially children, pedestrians and cyclists, from being killed or injured.
"Lower speeds make crashes less likely, and less severe when they do happen."
Mr Clinton said the 24% increase in casualties was worrying, but represented small numbers compared with casualties on 30mph roads.
He said: "Road deaths and serious injuries on Britain's roads as a whole increased in 2011 after consistently falling for many years.
"We need to understand why and to ensure that sufficient resources are devoted to road safety to make sure that one year's increase does not turn into a long term trend."
Institute of Advanced Motorists director of policy Neil Greig said: "The road safety evidence on 20mph areas now seems very mixed and contradictory.
"The IAM has always expressed concern that such areas were being seen as a magic bullet to stop all accidents when this had never been clearly proven.
"Traffic calming is popular when done well and in consultation with local residents. In our view the main benefits of 20mph zones are health and environmental improvements. The jury is still out on their wider road safety success."