Yahoo, one of the world’s most popular web portals and search engines, has said that it has identified a system breach that occurred in August 2013, involving data associated with more than one billion accounts.

The company said it believed this incident was probably distinct from the breach it disclosed in September, when information associated with at least 500 million user accounts was stolen from its network in 2014.

Yahoo, which is being acquired by Verizon, said an unauthorised third party had stolen the data in the latest breach announced and that it was working closely with law enforcement.

Yahoo said the stolen user account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords and, in some cases, encrypted or unencrypted security questions and answers.

Payment card data and bank account information were not stored in the system believed to be affected, the company said.