The breach "will unfortunately lead to a lot of hacking, targeted phishing and doxxing", Alon Gal, co-founder of Israeli cybersecurity-monitoring firm Hudson Rock, wrote on LinkedIn.
He called it "one of the most significant leaks I've seen".
Twitter has not commented on the report, which Gal first posted about on social media on December 24, nor responded to inquiries about the breach since that date.
It was not clear what action, if any, Twitter has taken to investigate or remediate the issue.
Reuters could not independently verify the data on the forum was authentic and came from Twitter.
Screenshots of the hacker forum, where the data appeared on Wednesday, have circulated online.
Troy Hunt, creator of breach-notification site Have I Been Pwned, viewed the leaked data and said on Twitter that it seemed "pretty much what it's been described as".
There were no clues to the identity or location of the hacker or hackers behind the breach.
It could have taken place as early as 2021, which was before Elon Musk took over ownership of the company last year.
Claims about the size and scope of the breach initially varied with early accounts in December saying 400 million email addresses and phone numbers were stolen.
A major breach at Twitter might interest regulators on both sides of the Atlantic.
The Data Protection Commission in Ireland, where Twitter has its European headquarters, and the United States Federal Trade Commission have been monitoring the company for compliance with European data protection rules and a US consent order respectively.
Messages left with the two regulators were not immediately returned on Thursday.
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