A London council hit by a major cyber attack this week has confirmed data was accessed and copied by hackers.
Kensington and Chelsea and Westminster Councils initiated emergency plans after the town halls were targeted by online criminals on Monday.
Hammersmith and Fulham is also understood to have been impacted as the local authorities share some systems.
The incident is being investigated by the National Crime Agency and the National Cyber Security Centre (NCSC).
A Kensington and Chelsea Council spokesman said on Friday: "We have now obtained evidence on our systems that shows some data has been copied and then taken away.
"At this moment in time, we believe the breach only impacts historical data.
"It is important to say we still have access to this information, it has not been stolen, but it is possible it could end up in the public domain.
“As a priority we are checking if this contains any personal or financial details of residents, customers, and service users – but this will take some time."
The Information Commissioner has been informed of the incident.
Elizabeth Campbell, leader of Kensington and Chelsea Council, said: “As soon as I was informed there might be a data breach impacting residents, I asked officers to make sure we tell people at the earliest possible opportunity.
“Even if we do not yet have the full picture of what data has been copied, and whether that does include any personal details, it is the right thing to do.
“All I know is - as a resident myself - I would want to know this information as soon as possible and then be able to make my own choices, follow advice, and protect myself if I think necessary.”
It has been suggested that the attack originated at Kensington and Chelsea Council, which shares some IT systems with its neighbouring local authorities.
A memo said the town hall acted “swiftly” to protect its own network but lost access to critical services, such as its call centre, for a several hours.
Other London councils have been urged to warn staff not to open suspicious emails, click on unexpected links in emails or verify unusual requests.
Earlier this year, Hackney Council said it was still dealing with the fallout from a cyber-attack five years ago.
Criminals accessed and encrypted some 440,000 files belonging to the local authority.
In July 2024, the Information Commissioner's Office said the local authority was not without blame as it criticised it for failing to effectively implement sufficient measures to protect its systems.
