You have a general duty under the Health and Safety at Work Act 1974 (HSW Act) to ensure, as far as is reasonably practicable, the health, safety and welfare at work of your employees. You also have a duty under the Management of Health and Safety at Work Regulations 1999, to assess the risks to the health and safety of your employees.
Knowingly allowing an employee under the influence of alcohol or substance misuse to continue working constitutes a breach of the Health and Safety at Work Act. A Breach of health and safety legislation is deemed to be a criminal offence.
Your employees are also required to take reasonable care of themselves and others who could be affected by what they do at work.