Local authorities said they were taking action after a data breach by the revenues and benefits software supplier Capita.
Councils including Colchester and Coventry city councils, South Staffordshire and Adur DCs and Worthing BC were reportedly affected.
The Information Commissioner's Office, the data protection regulator, said several councils had reported the incident and it was ‘assessing the information provided'.
It follows a Cyber attack on the firm in March that put files stored by councils in a cloud-based data storage system at risk for which a group of Russian hackers claimed responsibility.
Colchester's chief operating officer, Richard Block, said: ‘The privacy and security of personal information is paramount, and we are extremely disappointed that such a serious data breach by one of our contractors has occurred.'
A statement from Capita read: ‘Capita is working closely with all appropriate regulatory authorities, and with customers, suppliers and colleagues to notify those affected and take any remaining necessary steps to address the incident.'
