Small samples of the hacked data suggest it contains sensitive personal information, prompting warnings for residents to be vigilant against scams and unexpected contact.
Westminster City Council and Hammersmith and Fulham Council, which share some services, were also impacted, and all three are collaborating with the National Cyber Security Centre and law enforcement.
The council is currently checking accessed files, prioritising those belonging to vulnerable individuals, a process expected to take several months.
Council leader Elizabeth Campbell confirmed they immediately informed residents of the data theft and are working to identify and directly contact those at specific risk.