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.
This breaking news story is being updated and more details will be published shortly. Please refresh the page for the fullest version.