By politics.co.uk staff
Drug-driving is to be made an offence with its own fines and sentencing guidelines.
Police will be given the power to test suspected motorists' saliva to check whether they have been taking substances which could impede their ability to drive safely.
Similar legislation has already been passed in countries including Australia, Spain and Germany.
"We want to do for drug driving what drink driving laws have done for driving under the influence of alcohol," Mr Cameron said overnight.
"That's why we're doing what we can to get drugalysers rolled out more quickly. And this week we'll publish a new drug-driving offence so that driving under the influence of drugs itself is a crime, just like it is for drink-driving."
Police would initially test motorists who have been driving erratically for drink-driving and would then have the option of adding a drug test to the roadside test.
It is not yet clear which drugs would be included under the offence. The impact of individual drugs is being assessed by a panel of scientists reporting back to the Home Office.