A 2013 hack affected all three billion accounts at Yahoo, triple the original estimate, the online giant's parent company said on Tuesday, following a new analysis of the incident.
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.
The purchase price was cut following revelations of two major data breaches at Yahoo.
In addition to the 2013 breach, Yahoo said that hackers in 2014 stole personal data from more than 500 million of its user accounts.
The US Justice Department charged two Russian intelligence operatives and a pair of hackers over one of the attacks, which had apparent twin goals of espionage and financial gain.
Canadian authorities this year arrested Karim Baratov, 22, an immigrant from Kazakhstan, on a US warrant.
US authorities allege Russian intelligence agents hired Baratov and another hacker to carry out attacks on Yahoo from 2014 to 2016.
The attacks at Yahoo and Equifax have raised troubling questions about online security and data theft which may be used for fraud or espionage purposes.
Earlier Tuesday, a White House cybersecurity aide said officials were looking at using new types of identifiers online which would eliminate the use of social security numbers, which were leaked in the Equifax incident.
"I feel very strongly that the social security number has outlived its usefulness," Rob Joyce said.
For years, social security numbers have been used by Americans to open bank accounts or establish their identity when applying for credit. But stolen social security numbers can be used by criminals to open bogus accounts or for other types of identity theft.