As an adult worker, you are entitled to a rest break where your daily working time is more than six hours.
Details of this rest break, including the duration, can be regulated by a collective or workforce agreement.
If there is no such agreement, the rest break will be for an uninterrupted period of not less than 20 minutes and you are entitled to spend that break away from your workstation.
The break must be during the working day (not at the beginning or end) but otherwise can be at a time specified by the employer.
A period of 'downtime' when you are allowed to stop working but required to remain in contact with the employer is not a rest break, even if it turns out at the end of the break that it was uninterrupted. For it to be a proper break, you must know at the start of the break that it is to be uninterrupted by work duties.
Your employer is not allowed to group together lots of shorter bursts of time to make up 20 minutes. It must be a continuous twenty minute break.
Where your work is monotonous or the work-rate is predetermined, you have a separate right to adequate rest breaks (Regulation 8 of the Working Time Regulations). This is on top of the basic 20-minute break.
There are also separate rules about rest breaks where you operate a computer at work, found in the Health and Safety (Display Screen Equipment) Regulations 1992. In particular, regular breaks away from equipment are essential to combat the risk of repetitive strain injury.
The Working Time Regulations covering daily rest breaks do not apply where a worker's working time is unmeasured.
Mobile workers in road and rail transport have different rules. They must be given 'adequate rest'. Also, HGV and PSV road transport drivers have better entitlements through the Road Transport (Working Time) Regulations and the Tachograph Regulations that govern driving hours.