UP to one million people in the UK could be driving while under the influence of drugs, fleets are being warned amid new calls for laws to crack down on the problem.

Amid efforts to introduce new laws to tackle drivers taking to the road after using illegal drugs, experts warn that legal drugs also pose a major problem to road safety.

Research by the RAC Foundation has shown that drivers taking anti-depressant drugs could have their driving ability impaired because of side effects. Sue Nicholson, head of campaigns at the RAC Foundation, said: 'I am certain that this would include many fleet drivers as depression affects people in all professions.'

Research from the Department for Transport into some of the newer anti-depressants has highlighted a need for further studies into drugs and driving.

Edmund King, executive director at the RAC Foundation agreed. He said: 'There is considerable evidence that older generation anti-depressant drugs and tranquillisers have an adverse effect on driving. This can increase the risk of accidents but not enough work has been done on the relationship between the newer forms of medication and driving.'

However, Nicholson believes that fleet managers could implement procedures, which would protect company drivers.

She said: 'Many people don't want to discuss mental health as they think there is a stigma attached so wouldn't approach their employer. The best thing is to have a standard procedure put in place for all company drivers. This would give advice such as refraining from driving for a couple of days whilst on the medication.'

The RAC Foundation is already backing efforts to tackle drivers who use illegal drugs and drive, amid warnings it could be a greater problem than drinking and driving. Shadow Home Office minister Nick Hawkins MP has put forward a Bill that would give police officers more powers to test motorists at the roadside if they believe they have been taking illegal drugs.

Hawkins said: 'People who drive under the influence of drugs endanger not only themselves, but other road users. I very much hope that the Government will not block this Bill, which is very important for the safety of law-abiding drivers.'