A third party told Labour that he incident had resulted in a significant quantity of Party data being rendered inaccessible on their systems with an investigation now ongoing.
Supporters have been emailed to warn them information being stored by a third party may have been compromised.
The incident took place on 29 October with the incident being reported to the relevant authorities, including the National Crime Agency (NCA), National Cyber Security Centre (NCSC) and the Information Commissioner’s Office (ICO).
According to an email sent to members, the party’s own data systems were unaffected by this incident.
It is understood that the data involved includes information provided to the Party by its members, registered and affiliated supporters, and other individuals who have provided their information to the Party.
An NCA spokesman told The Metro: “The NCA is leading the criminal investigation into a cyber incident impacting on the Labour Party.
‘We are working closely with partners to mitigate any potential risk and assess the nature of this incident.’
An NCSC spokesman added: “We are aware of this issue and are working with the Labour Party to fully investigate and mitigate any potential impact.
‘We would urge anyone who thinks they may have been the victim of a data breach to be especially vigilant against suspicious emails, phone calls or text messages and to follow the steps set out in our data breaches guidance.
‘The NCSC is committed to helping organisations manage their cyber security and publishes advice and guidance on the NCSC website.’