The law says that most workers should not have to work more than 48 hours a week on average. This includes any overtime. However, you can choose to work more than 48 hours a week as long as you have agreed this with your employer. Your employer cannot force you to work more than 48 hours if you do not agree.
The rule that you cannot work more than 48 hours a week, does not mean that you can never work more than 48 hours in one week. The number of hours you can spend at work are usually averaged out over a period of 17 weeks. This is called the reference period.
To work out your average weekly working hours, you need to divide the number of hours you have worked by the number of weeks in the reference period. As long as this number is not more than 48, you are working within the limits that the law allows.
There are different rules for workers aged under 18, who must not normally work more than 40 hours a week.