Yahoo has said that all of its three billion user accounts were affected in a hacking attack dating back to 2013.
The company, which was taken over by Verizon earlier this year, said an investigation had shown the breach went much further than originally thought.
The stolen data did not include passwords in clear text, payment card or bank account data, it added.
Previously the internet giant had said “more than one billion” of its accounts had been hit.
Yahoo said that while its latest announcement did not represent a new “security issue” it was sending emails to all the “additional affected user accounts”.
The company added that it was “continuing to work closely with law enforcement”.