Previously slated to launch this autumn, the new drug-driving offence is due to come into force on 2nd March 2015, according to the Department for Transport, which said it will be timed to arrive as new equipment to test drivers for cannabis and cocaine at the roadside is expected to become available to the police forces.

Although police can already prosecute drivers if they are driving whilst impaired by drugs, including medicinal drugs, the new rules will mean it will be an offence to be over the specified limits for each drug whilst driving, as it is with drink-driving.

Earlier this year, the Government announced that it has approved recommended drug drive limits for 16 substances.

The DfT added today that the limits for the vast majority of medicinal drugs are above the normal doses. Unlike the existing “impairment” offence, the new law provides a medical defence for patients who are taking their medicine in accordance with instructions – either from a healthcare professional or printed in the accompanying leaflet – provided they are not impaired.

The new guidance announced today is intended to help advises general practitioners, doctors, nurses and health advisers on changes to the drug-driving offence and what it means for patients.

The guidance also advises patients who take legitimately supplied medicines to keep evidence with them in case they are stopped by police. This will help speed up any investigation where the medical defence is used and reduce the inconvenience to the patient.

If a driver tests positive they will be taken to a police station where a further evidential test will be taken. If this is positive it will allow police to prosecute a driver for being over the limit.

Roads Minister Robert Goodwill said: ‘The new guidance will help doctors, nurses and healthcare professionals explain the new drug-driving offence and reassure their patients that provided they take their medication in accordance with advice and are not impaired they can carry on driving just as they have always done.

‘The new drug-driving law will make it easier for the police to tackle those who drive after taking illegal drugs or abuse medicinal drugs whether they are on prescription or available over the counter.

‘This new offence will be introduced alongside major changes to drink-drive laws. Taken together, these will give police the tools they need to prosecute those who risk the lives of others through dangerous behaviour and make our roads safer.’

Meanwhile changes to drink-driving are being introduced as part of the Deregulation Bill and will remove the right for drivers who fail a roadside breath test to demand a blood or urine sample at the police station, due to concerns that drivers could have time to sober up. New mobile breath testing equipment is also expected to be approved early in 2015.

In response to the news, IAM director of policy and research Neil Greig said: 'This is good news although the regulations are complex and medical practitioners will need extra time and patience to explain and reassure drivers that they are unlikely to be affected. The key point is that no one taking prescription drugs in the way that their doctor tells them to should fall foul of the new drug-driving laws. We can now look forward to finally seeing drug testing rolled out across the country later this summer.’