Police forces are to be challenged in the high court over how they enforce speed limits on the roads.

The legal challenge follows the disclosure earlier this year of internal police guidelines covering England, Wales and Northern Ireland which recommend that drivers who broke the 30mph limit should not be prosecuted unless they were doing more than 35mph.

Transport 2000, a pressure group, has been given leave by the high court to challenge this 5mph "buffer zone".

The campaigners argue that the policy is illegal and want to see the 30mph limit strictly enforced. "This would send a signal to drivers that speeding kills and is a crime," said Transport 2000's director, Stephen Joseph.

An average of 10 people a day are killed on British roads and a further 100 injured. Government research shows that someone hit by a car at 35mph is more than twice as likely to be killed as someone hit at 30mph.

"We believe this edging up of the speed limit set by parliament will mean more people killed and seriously hurt on the roads," said Lynn Sloman, of Transport 2000.

"We are particularly concerned about child safety - nearly 100 children are killed or badly injured every week."

The speed enforcement policy was introduced by the Association of Chief Police Officers in February. It was designed to bring some consistency into the way speeding motorists are treated across England, Wales and Northern Ireland.

Ken Williams, chief constable for Norfolk and chairman of the association's traffic committee, said that the police were committed to catching speeding drivers, but would defend the guidelines in court.

"My association is committed to a strategy of contributing to the reduction of deaths and injury on the road through partnership with all those who share a common goal," said Mr Williams.

He added that he hoped to be talking to the Transport 2000 campaigners about their concerns. Transport 2000 is also arguing for tougher penalties for speeding in 30mph areas. At present guilty drivers face a fixed £60 fine and three penalty points on their licence. The group wants all drivers caught speeding to face court; police practice is to prosecute when in excess of 50mph.

Useful links:

• Transport 2000
• Royal Society for the Prevention of Accidents