Parents are monitoring their teenage children's driving with a spy-under-the-bonnet device that records any dangerous manoeuvre.

If a young driver brakes or accelerates aggressively, takes a corner too fast or pulls out suddenly to overtake, the device registers the incident and transmits the information back to the parent via the web.

The parent can log on and look at their child's safety score for each trip within 20 minutes of its completion. They can check what mistakes are being made and when the car exceeds 70mph.

Young drivers receive a 25 per cent discount on their insurance premium for agreeing to have the device fitted. The average teenage male driver, who pays £2,000 for third party insurance, would save £500. Participants are rewarded with shopping vouchers for each month of safe driving.

A six-month trial by Staffordshire County Council involving 40 young drivers found that the number of high-risk manoeuvres fell by 58 per cent after information from the device was fed back to parents. Simply having the device fitted made little difference to the teenagers but their driving improved sharply when they knew that their parents were watching and could study their safety scores. Fuel consumption fell by an average of 10 per cent.

More than 14 young drivers and their passengers are killed every week. Men aged 17 to 20 are almost ten times more likely to be killed or seriously injured behind the wheel than those aged 40 to 59.

Staffordshire and Warwickshire county councils plan to install the technology, GreenRoad Safety Center, on 1,000 teenage drivers' cars. Another 20 councils are considering offering the devices.

The system includes a dashboard unit containing green, yellow and red lights to give drivers instant feedback on their risk level. If a driver commits three high-risk manoeuvres within 10 minutes, the unit will show red for at least 20 minutes.

Drivers are given an overall score, based on the number of high-risk manoeuvres in ten hours of driving. They can compare their performance with the average of all participants.

Parents are encouraged to discuss the results with their children. Drivers with particular problems are offered extra coaching by instructors. Those who fail to show improvement may lose their insurance discounts the following year.

To make the system more acceptable to young drivers, it does not tell parents where they are driving nor how fast they are going if they remain below 70mph. The system does possess the capacity to record speed and location.

Some parents in the trial said that they would like to be informed when their child exceeded the speed limit, especially in residential areas.

Adrian Hide, Staffordshire's road safety manager, said: “This is like a virtual driving instructor providing feedback and encouragement.”

Andrew Howard, head of road safety at the AA, said: “Young drivers know how to drive safely, it's just that they choose not to. With a parent keeping an eye on them, they are less likely to show off.

“The only risk is that they may choose to travel in friends' cars which do not have the device.”

He said that some teenagers would not like to be monitored even in return for insurance discounts and rewards. “But parents are usually paying for the car and lending money for the insurance, so that should help persuade the teenager to accept it. By the time they become financially independent, most will have got past the crucial high-risk period.”