Drug testing resembles an alcohol breath test

The problem appears to be on the increase - the RAC figure was 18% three years ago.

About 200 people a year die because of drug driving.

Last month police were given new powers to test drivers for drugs.

But the roadside 'drugalyser' has not yet had Home Office approval and will not be available until next year at the earliest.

The Railway and Transport Safety Bill gave police the power to scan saliva samples.

If the roadside test is positive, it is repeated at a police station in the same way as for alcohol.

The idea is already being piloted in France and Australia.

Earlier this year, police in Avon and Somerset held a month-long campaign against drug driving in which suspects were asked to perform "impairment tests" - similar to old-style drink driving checks.

Drivers were asked to carry out a set of physical exercises designed to test their co-ordination.