The attack was discovered on the 24th of November.
Investigations have revealed that some of the resident data copied is likely to contain sensitive data and personal information.
The council has written to over 100,000 households with guidance on what to do if they are worried about the breach.
Between June and September last year, Kensington and Chelsea intercepted and isolated over 113,000 phishing attempts.
Bosses say they spend £12 million a year on Digital, Data, and Technology.
What the council says;
Our investigation is ongoing and will take several months, due to the complex nature of the attack and the data involved, and the need to restart many of our systems.
We are committed to sharing accurate updates as soon as new information becomes available.
We will write to people if they are impacted by the data breach.
We continue to work closely with law enforcement and cybersecurity agencies here and internationally to investigate the incident.
We understand this is a worrying situation, and we thank residents, local partners, and our own staff for their patience and cooperation.
We will be establishing a cyber recovery team in the coming weeks, and further updates will be available on our website. Our staff are here and ready to help.
