London councils have activated "emergency plans" after being hit by a cyber attack.
Westminster and Kensington and Chelsea town halls said they are urgently working with the National Cyber Security Centre following a hack on Monday.
Other local authorities in the capital are also believed to have been targeted, including Hammersmith and Fulham council.
Westminster and Kensington and Chelsea said shared IT systems and phone lines have been affected.
The source of the attack, and its scale, have not yet been fully established.
A Kensington and Chelsea Council spokesman said: “We identified the issue quickly on Monday and are working with cyber specialists and the National Cyber Security Centre to protect data and restore services.
“We’ve activated business continuity and emergency plans to ensure we are still delivering critical services to residents, focusing on supporting the most vulnerable.
“We are investigating the cause of the incident and will provide more information when it is available. We would like to apologise for any disruption and thank residents for their patience as we work to bring systems back online safely.”
A Westminster Council spokesman added: “At this stage it is too early to say who did this, and why, but we are investigating to see if any data has been compromised – which is standard practice.
“Our IT teams worked through the night yesterday and a number of successful mitigations were put in place, and we remain vigilant should there be any further incidents or issues.”
The Information Commissioner has been informed of the incident.
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.
Affected systems are unlikely to be up and running before the end of this week.
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.
The October 2020 hack left a lasting impact on services.
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.
