It depends. The Working Time Regulations define working time as any period during which the worker is:

• working;
• at the employer's disposal; and
• carrying out the worker's activities or duties.

You are likely to be “working” while you are on standby if your employer can dictate where you must stay (for example, at the workplace, in your home, or inside a fixed geographical area) and can require you to respond so as to provide services immediately in case of need.

In an important case in 2018, Ville de Nivelles v Matzak,  the European Court of Justice ruled that volunteer firefighters were working when on “standby” waiting at home for call outs. This was on the basis that they were required by the fire authority to stay at home and to respond to any call out within eight minutes.

Time during which your employer requires you to remain contactable but does not limit where you are allowed to go will not be working time.