What are the maximum hours I can be expected to work over an average working day?

Workers covered by the Working Time Regulations 1998 must not be required to work more than 13 hours per day and must not be required, against their wishes, to work an average of more than 48 hours a week. Workers may agree in writing to work more than an average of 48 hours per week (an “opt out agreement”) but can withdraw their agreement at any time.  

The hours that an individual works in a week are averaged out over what is termed a 'reference period'. The usual reference period is 17 weeks 

There are exceptions to the formula for calculating the reference period for certain industries and occupations, and where the employer agrees a collective agreement or workforce agreement with employee representatives. 

There are different rules for mobile transport workers and tighter rules for young workers aged 16 and 17.