Yahoo has said more than one billion user accounts may have been affected in a hacking attack dating back to 2013.
The internet giant said it appeared separate from a 2014 breach disclosed in September, when Yahoo revealed 500 million accounts had been accessed.
Yahoo said names, phone numbers, passwords and email addresses were stolen, but not bank and payment data.
The company, which is being taken over by Verizon, said it was working closely with the police and authorities.
Yahoo said in a statement that it "believes an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts."
The breach "is likely distinct from the incident the company disclosed on September 22, 2016".
However, the three-year-old hack was uncovered as part of continuing investigations by authorities and security experts into the 2014 breach, Yahoo said.
Account users were urged to change their passwords and security questions.