Facebook has said it now believes up to 87 million people’s data was improperly shared with the political consultancy Cambridge Analytica.
The BBC has been told that about one million of them are UK-based.
The overall figure had been previously quoted as being 50 million by the whistleblower Christopher Wylie.
The details were revealed in a blog by the tech firm’s chief technology officer, Mike Schroepfer.
It was published several hours after the US House Commerce Committee announced that Facebook’s founder, Mark Zuckerberg, would testify before it on 11 April.
The tech firm has faced intense criticism after it emerged that it had known for years that Cambridge Analytica had harvested data from millions of its users, but had relied on the London-based firm to self-certify that it had deleted the information.
Channel 4 News has since reported that at least some of the data in question is still in circulation despite Cambridge Analytica insisting it had destroyed the material.
Mr Schroepfer also detailed new steps being taken by Facebook in the wake of the scandal.
- a decision to stop third-party apps seeing who is on the guest lists of Events pages and the contents of messages posted on them
- a move to stop users being able to search for others by typing a phone number or email address into the search box. The firm said the feature had been abused by malicious actors who had scraped the information from elsewhere and were trying to identify who it belonged to
- a commitment to only hold call and text history logs collected by the Android versions of Messenger and Facebook Lite for a year. In addition, Facebook said the logs would no longer include the time of the calls
- a link will appear at the top of users’ News Feeds next week, prompting them to review the third-party apps they use on Facebook and what information is shared as a consequence
The documents are longer than the existing editions in order to make the language clearer and more descriptive.