The code, which is expected to come into force in mid-April this year, recommends that employers should only use drug testing if it can be justified on safety grounds. This “justification on safety grounds” has yet to be clarified.

According to research published by the TUC and the Institute for the Study of Drug Dependence in 1998, alcohol misuse costs employers £2 billion a year, and drugs misuse a further £800 million. Around a quarter of workplace accidents involve workers who have been drinking, the research also found.

Cynthia Atwell, an occupational health consultant and former HSE member, believes some employers should be allowed to ask employees to take routine drugs tests.

She explains: “If the employee caused an accident where someone was killed or injured, the employer would be liable under the Health and Safety at Work Act because managers have a duty of care to oversee the health and safety of others.”

As the law stands, employers are only allowed to ask an employee to take a test if they have reason to believe that he or she is under the influence of drugs. The employee is under no obligation to comply.

Kieron Hill, manager of the advice service at employment law consultants Peninsula Business Services, says: “An employer has to make a reasonable judgement based on an employee's behaviour as to whether to ask them to take a test. In the case of staff working with machinery, an employer does have the right to send someone home if they are acting strangely or their attention seems impaired.”

“Employees are allowed to refuse to take the tests. However, if submitting to tests is part of their contract, the company can fire them for breach of contract,” Hill adds.

The Health and Safety Executive (HSE) has recently begun work to establish the scale and impact of drug misuse on health and safety at work. This will contribute to establishing a database on drug-related accidents at work.

At present, there are only two pieces of legislation dealing specifically with drug use in the workplace. The Transport and Works Act 1992 makes it a criminal offence for certain railway workers to be unfit for work through drugs or drink. The Misuse of Drugs Act 1971 makes it an offence to possess, supply, offer to supply or produce controlled drugs without authorisation.

This issue is far from clear-cut, experts say, because employers are not just dealing with workers who are visibly dependent on drugs, but also with those who use them on a recreational basis.

From an employer's perspective, the effects of substance abuse by employees include a rise in sickness and absenteeism, increased risk of accidents, and a deterioration in the quality of work.

Kieron Hill continues: “The problem lies in the fact that it is difficult to recognise when people are under the influence of drugs. Behaviour can change for a number of reasons whether it is due to illness, prescribed drugs or illegal ones.”

The Chartered Institute of Personnel and Development urges employers to develop a workplace policy that addresses drug and alcohol misuse. The aim should be to raise awareness of these issues and to encourage employees with problems to seek help, the CIPD says.

Tricia Jackson, a personnel consultant specialising in employment law, states in her drugs and alcohol policies book, written for the CIPD: “When looking for signs of drug or alcohol misuse, employers should note that some of the symptoms are very similar to those of a range of medical conditions, notably diabetes and epilepsy.”

“The correct response is to make a full investigation of all the circumstances, including gathering of medical evidence before making any decisions. The next step would be to obtain medical advice – with the employee's consent – in order to ascertain whether there is an underlying health problem.”

Last month, speaking at the third annual conference for Combating Drugs and Alcohol in the Workplace, Keith Hellawell, the UK's 'drugs czar', explained: “What is required for a successful policy is comprehensive dialogue and consultation between unions, management and the workforce, so everyone has a part in the formulation of the policies.”

The introduction of a drugs and alcohol policy will impact on employment contracts, especially disciplinary rules, and terms and conditions. Employees must be notified of any change, or their bosses run the risk of being hauled before an employment tribunal in the event of a drugs-related dismissal.

Currently, employers are entitled to ask potential employees to submit to tests, but cannot conduct tests of their own without the consent of employees. Any attempt to do so without permission would constitute a criminal offence.