He made the comments after official figures showed that half of all motorists who the police tested between June and July this year had banned substances in their systems.
From 14 June to 15 July, 57 per cent of 1,962 motorists tested by the police were found to be driving under the influence of banned substances.
In contrast, around 10 per cent of the 36,675 motorists the police tested for excess alcohol were over the drink-driving limit.
Jeremy Sirrell said: “The reason the failure rate for banned substances is so much higher than that for alcohol seems to be that the police are mainly testing motorists for banned substances who appear impaired but who have passed a breathalyser test.
“The police are unlikely to test a driver who has failed an alcohol breathalyser test for banned substances unless they have a particular reason to do so, such as finding drugs in the car.
“This means that the real figure for drug driving could be much higher than it appears from the official statistics.”
He said that another reason that so many motorists tested by the police for banned substances failed the test is that the Government has set the level of drugs allowed in the bloodstream at an extremely low level.