Smoking cannabis within three hours of driving could almost double the risk of a serious crash, research suggests.

A review of nine studies found that drivers were more likely to be involved in a collision with another car after smoking the class B drug.

Figures show there are around a million users of cannabis in the UK aged 16 to 24 and around two million in the 16 to 59 age group.

Research published in the British Medical Journal found cannabis use led to a “near doubling of risk of a driver being involved in a collision resulting in serious injury or death”.

The researchers, from Dalhousie University in Canada, said, however, that the impact of cannabis consumption “on the risk of minor crashes remains unclear”. Previous studies suggested cannabis impairs a person’s mental abilities and the ‘motor tasks’ needed for safe driving, increasing the risk of a crash.

“The results also accord with recent data for collisions that point to the increasing presence of drugs other than alcohol, especially cannabis and depressants of the central nervous system, in injured and fatally injured drivers.”