Drivers who spot hidden speed cameras will be able to alert other vehicles within three seconds with the help of a dashboard gadget. They will no longer need to flash their headlights to oncoming drivers but will simply press a button on a satellite-positioning device.

The device, which exploits a loophole in the law, transmits the location of the speedtrap to a processing centre. The information is relayed to other drivers who have installed the same equipment. A car travelling 300 yards behind the driver who first spots the trap would receive the warning in time to slow down before the camera.

The device emits an audible alarm and a colour-coded warning telling the driver how frequently the speed trap has been reported. It also displays the car’s speed, the speed limit and the distance to the camera.

Most existing speedtrap warning devices use satellite positioning to compare the vehicle’s location with a database of known camera sites. But the database is often out of date and drivers can still be caught out by mobile camera units, which change their location frequently and sometimes operate covertly.

Road safety groups said that the new device would undermine the ability of police to enforce the limit because drivers would be able to speed with very little risk of being caught. More than 50,000 drivers in France already have the new device. In September they reported 27,000 traps.

If a driver repeatedly presses the button in places where no one else reports a camera, his transmissions will be blocked.

Novus, the company behind the device, which is known as Mini Coyote, is taking advantage of a legal grey area. Police forces sometimes give warnings that drivers could be prosecuted for perverting the course of justice if they try to alert other drivers to speed traps. But there have been very few successful prosecutions.

The Government has said it intends to outlaw devices that detect radar or laser systems used by speed cameras. But when contacted by The Times yesterday, the Department for Transport appeared unaware of the existence of the Mini Coyote and unsure how to respond to it. A spokes-woman said: “The police do need the ability to carry out unannounced enforcement with mobile cameras.”

Robert Gifford, director of the Parliamentary Advisory Council for Transport Safety, said: “It should be made clear to drivers who are thinking of buying these devices that they could be prosecuted for perverting the course of justice.”

But Edmund King, president of the AA, said: “We think it would be extremely difficult to prosecute someone for perverting the course of justice if they had warned another motorist of speed cameras ahead.”

Nigel Carter, from Novus, said: “This is actually a road-safety device because it will help prevent accidents caused by drivers stamping on the brakes when they spot a camera too late. As far as we can see, there is nothing illegal in the unit.”

Pulling a fast one

— In 1912 the AA employed 950 “cycle scouts” to patrol roads and alert members to speed traps. The AA car badge helped scouts to identify members

— The AA advised members to slow down if scouts or breakdown patrolmen did not salute as they passed, thus allowing the AA to avoid prosecution for helping drivers evade the law

— Until 1998 speed camera radar detectors had been thought to be illegal under the Wireless and Telegraphy Act 1949

— The Road Safety Act 2006 gave the Government the power to ban radar and laser speed camera detectors