The disclosure from Verizon, which acquired Yahoo's online assets earlier this year, revised upward the initial estimate of one billion accounts affected.

The statement said the estimate is based on "new intelligence" following an investigation with the assistance of outside forensic experts into the incident in August 2013.

"While this is not a new security issue, Yahoo is sending email notifications to the additional affected user accounts," said a statement issued by Verizon's internet unit known as Oath.

"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 company is continuing to work closely with law enforcement."

The Yahoo breach was already believed to be the largest ever in terms of numbers of users affected. But a recently disclosed breach by credit agency Equifax is seen as potentially more damaging because of the sensitivity of the data leaked.

- Yahoo brand lives on -Although Yahoo is no longer an independent company -- its financial holdings are in a separate company now called Altaba -- Verizon has continued to operate the Yahoo brand, including its email service and a variety of news and entertainment websites.

Oath includes the Yahoo internet operations along with those of another former internet star, AOL.

"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, the company's chief information security officer.

"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."

Yahoo, which was once one of the leading internet firms, sold its main online operations to Verizon in a deal that closed in June for $4.48 billion.