Facebook believes the data of up to 87 million people was improperly shared with the political consultancy Cambridge Analytica – many more than previously disclosed.
The BBC has been told that about 1.1 million of them are UK-based.
The overall figure had been previously quoted as being 50 million by the whistleblower Christopher Wylie.
Facebook chief Mark Zuckerberg said “clearly we should have done more, and we will going forward”.
During a press conference he said that he had previously assumed that if Facebook gave people tools, it was largely their responsibility to decide how to use them.
But he added that it was “wrong in retrospect” to have had such a limited view.