The UK GDPR contains a general requirement for a personal data breach to be notified by the controller to the ICO, and for more serious breaches to also be notified to affected data subjects. A "personal data breach" is a wide concept, defined as any "breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to, personal data transmitted, stored or otherwise processed" (Article 4).

The controller must notify a breach to the ICO without undue delay, and where feasible, not later than 72 hours after having become aware of it, unless the controller determines that the breach is unlikely to result in a risk to the rights and freedoms of natural persons. When the personal data breach is likely to result in a high risk to natural persons, the controller is also required to notify the affected data subjects without undue delay (Article 34).

Where the breach occurs at the level of the processor, it is required to notify the controller without undue delay upon becoming aware of the breach (Article 33(2)).

The notification to the ICO must include where possible the categories and approximate numbers of individuals and records concerned, the name of the organisation’s data protection officer or other contact, the likely consequences of the breach and the measures taken to mitigate harm (Article 33(3)).

Controllers are also required to keep a record of all data breaches (Article 33(5)) (whether or not notified to the supervisory authority) and permit audits of the record by the ICO.

Breaches in the United Kingdom can be reported to the ICO's dedicated breach helpline during office hours (+44 303 123 1113). Outside of these hours (or where a written notification is preferred) a pro forma may be downloaded and emailed to the ICO.