Motorists will be compelled to take part in the roadside tests

Previously drivers could choose to take part in the impairment tests, but now refusal is an offence in the same way as failure to provide a breath test.

A code of practice explains how the tests, such as asking drivers to stand on one leg, should be carried out.

The maximum penalty for drug-driving - which covers illegal and prescription drugs - is six months in prison.

The tests are designed to show whether a driver's balance, judgement and ability to do two things at once have been affected by drugs.

DRUG DRIVE TESTS The five tests police use to check if a driver is under the influence In graphics

Drivers could be asked to estimate when 30 seconds has passed and touch a finger to their nose.

Officers will also use a gauge to detect how much someone's pupils are dilated.

While detection equipment is also being developed, to be ready next year, the Department of Transport says it will only show if drugs are present but not whether driving ability has been impaired.

Road safety minister David Jamieson said: "Drug-driving puts lives at danger and is as irresponsible as drink-driving.

"Drivers should never get behind the wheel when they're unfit to drive."

The Code of Practice details the tests to be undertaken, how they should be administered, the kind of observations that may be made and the inferences that may be drawn.

The Department for Transport, following a three-year study, said 18% of people who died in road traffic accidents where known to have been driving with drugs in their systems.

'Taboo subject'

This is a six-fold increase compared to a 1989 study which showed only 3% of a random sample of road traffic accident driver fatalities were known to have drugs in their systems.

SEE THE POLICE FORM Impairment test form as used by police(227k) Most computers will open PDF documents automatically, but you may need to download Adobe Acrobat Reader. Download the reader here

The new police powers were welcomed by RAC Foundation executive director Edmund King, but pressed the need for more public information about the dangers of drug-driving.

"For too long the real problem of drugs and driving has been a taboo subject.

"We are delighted that the situation is to be clarified and that the police will have new powers to tackle drug-drivers.

He added: "Many motorists are surprisingly ignorant of the dangers posed by drug-driving."