An analysis of the country’s first city-wide 20mph speed limit has cast doubt over widescale plans to introduce it to other towns and cities.

The speed limit in Portsmouth was cut from 30mph to 20 on all residential streets in the city but a study found that since the limit was lowered the number of people killed or seriously injured in road accidents has actually risen.

 The new limit, which is not enforced by more speed cameras or road humps, has seen the number of people killed or seriously injured on the roads in Portsmouth go up from 18.7 people per year to 19.9.

Similar schemes have already been introduced in Newcastle, Oxford, Edinburgh and Bristol in recent years and many more towns and cities are considering lower limit areas. However motoring groups say the findings in Portsmouth must question whether the 20mph is making any difference.

Paul Watters, head of public affairs at the AA, said: “By just whacking up signs everywhere you are not going to change things dramatically. We support targeted and tailored 20mph zones where they are really needed, not a blanket implementation across a whole city.”

The coalition government is believed to be in favour of 20mph zones. Transport Minister Norman Baker recently said that for a child being hit at 30mph and 20mph is the difference between life and death.