The Conservatives have called for more research into the dangers of drug-driving.

Shadow Home Office minister Nick Hawkins expressed concern after a survey by Durham Police showed an increase in drug-taking accidents.

"The evidence suggests that at least one in five road deaths are linked to drugs," he warned.

But despite the risks, Hawkins said that the government was ignoring the problem.

"Unforgivably, the Home Office seems to have its head in the sand. It does not collect drug-driving statistics adequately, and does not require drivers to be tested properly. It is vital that we discover the scale of the problem if we are to tackle it effectively," he said.

Currently driving while unfit due to the influence of drugs is a criminal offence which can be punished with a year's ban, a heavy fine or a spell in prison.

But the Conservatives warn that roadside tests lag way behind the breathalysers used to detect drink-driving.

Police must rely on basic co-ordination tests such as asking a driver to walk in a straight line which they cannot legally insist on, says the party. Alternatively the police can arrest suspects and take them to the police station for blood or urine samples, which may take six weeks to come back from the laboratory.

And as the government moves to reclassify cannabis - which can impair coordination, visual perception and vigilance - the Conservatives warned that the measure could make things even worse.

"The message being given to those in their late teens, many of whom are starting to drive, is that it is now fine to use cannabis," said Hawkins.

"The vast majority of teenagers will not be interested in the subtleties of ministers saying that it has been reclassified lower but is still an offence. The message they get is simply that it's okay to take.

"This will inevitably lead to much greater use. More young people will start driving under the influence of cannabis, and this in turn will lead to a substantial increase in the number of accidents," he said.