Facebook has said more users may have been affected by the Cambridge Analytica data leak than previously thought.
Up to 87 million people may have had their personal information taken from Facebook, its chief technology officer Mike Schroepfer wrote in a blog post.
Previous estimates thought just over 50m users had been affected by Cambridge Analytica accessing data.
Most of the 87 million people whose data was shared with the political consultancy, which worked on Donald Trump’s presidential campaign, were in the United States, he added.
A total of 70,632,350 users in the US were affected while 1.79 million users were in the UK.
Mr Schroepfer did not detail how Facebook came to determine the higher estimate of 87 million users affected.
He said Facebook would tell people if their information may have been improperly shared with Cambridge Analytica.
Facebook is taking steps to restrict the personal data available to third-party app developers, he added.
Facebook chief executive Mark Zuckerberg admitted mistakes were made and said his company has not taken a broad enough view of its responsibility in the world.
He called it a “huge mistake, it’s my mistake”.
During a conference call to reporters he said he wishes he could snap his fingers and solve everything in three to six months but said it would take many years to fix the company’s problems.
“These are big issues” he said, adding that the company will have “turned a corner” on a lot of the issues by the end of the year.
From Monday, 9 April, Facebook users will be given a link at the top of their news feed so they can see what apps they use and the information they have shared with those apps.
As part of that process people will be told if their information may have been improperly shared with Cambridge Analytica.
The largest social media company in the world – with a billion users – has seen its shares plunge as it faces anger from users, advertisers and politicians after a series of “fake news”, election-meddling and privacy scandals.
Mr Zuckerberg will testify over the matter next week before the US House Energy and Commerce Committee, the panel said on Wednesday.
Mr Zuckerberg has so far resisted calls from the British government to face its digital committee over the data leak, which includes allegations about the site being used to influence the Brexit referendum.
Facebook shares were down 1.4% on Wednesday to $153.90 (£108).
They are down more than 16% since the Cambridge Analytics scandal broke on 19 March.
The British-based consultancy has denied any wrongdoing, saying it engaged a university professor “in good faith” to collect Facebook data in a manner similar to how other third-party app developers have harvested personal information.
Nigeria announced on Monday it is joining the US and Britain in investigating improper involvement by Cambridge Analytica in its 2007 and 2015 elections.