Yahoo has admitted that all 3bn of its accounts were hacked in a 2013 hack, which is three times more than its earlier estimate of the largest data breach in history.
The hack, which took place in August 2013, was first disclosed by Yahoo in December last year. At the time, the group said as many as 1bn of its users' information may have been compromised.
However, the company said overnight that it has "recently obtained new intelligence and now believes, following an investigation with the assistance of outside forensic experts, that all Yahoo user accounts were affected by the August 2013 theft".
While this is not a new security issue, Yahoo said it was sending email notifications to the additional affected user accounts.
"The investigation indicates that the user account information that was stolen did not include passwords in clear text, payment card data, or bank account information," the firm said, adding that it is continuing to work closely with law enforcement.
"Verizon is committed to the highest standards of accountability and transparency, and we proactively work to ensure the safety and security of our users and networks in an evolving landscape of online threats," said Chandra McMahon, chief information security officer, Verizon.
"Our investment in Yahoo is allowing that team to continue to take significant steps to enhance their security, as well as benefit from Verizon's experience and resources."