You cannot work more than 48 hours a week on average - normally averaged over 17 weeks. This law is sometimes called the working time directive or working time regulations.
You can choose to work more by opting out of the 48-hour week.
If you are under 18, you cannot work more than 8 hours a day or 40 hours a week.
You may have to work more than 48 hours a week on average if you work in a job:
where 24-hour staffing is required
in the armed forces, emergency services or police
in security and surveillance
as a domestic servant in a private household
as a seafarer, sea-fisherman or worker on vessels on inland waterways
where working time is not measured and you are in control, eg you are a managing executive with control over your decisions
Additional information on maximum weekly working hours can be found on GOV.UK's website.