A British data firm has been suspended by Facebook amid allegations it harvested personal details from more than 50 million users.
Cambridge Analytica played a key role in mapping out the behaviour of voters in the run-up to the 2016 US election and was also used during the EU referendum campaign earlier that year.
What is Cambridge Analytica alleged to have done?
It's alleged the firm was passed personal data from Facebook apps without the consent of the users. More than 50 million profiles were allegedly subject to data grabs in 2014.
How did it work?
Facebook's vice president and deputy general counsel Paul Grewal announced the suspension in a blog on the site.
He said in 2015 Facebook learned University of Cambridge professor Dr Aleksandr Kogan passed on data to a third party.
Dr Kogan had launched a Facebook app called thisisyourdigitallife, which offered personality predictions and was downloaded by 270,000 people.
Facebook claimed he handed over information on app users - which included details such as their likes and location - to Cambridge Analytica and others.
Despite assurances at the time that the data had been destroyed, the social media company was informed in recent days that this had not happened, prompting the suspension of the firm on suspicion it had flouted privacy rules.
Dr Kogan was also allegedly involved in a data grab of more than 50 million profiles, using his company called Global Science Research (GSR) to accrue information.
It offered users a small amount of money to complete a survey on the condition they consented to share personal details through Facebook.
This, it is claimed, allowed researchers to build personality and psychological profiles on millions of users.
Cambridge Analytica could then tailor specific political adverts to small groups of people, already knowing what their likes and interests were, it is alleged.
How did the allegations arise?
A whistleblower called Chris Wylie, a former research director at Cambridge Analytica, told Channel 4 News about the breach.
Mr Wylie told the broadcaster: "Imagine I go and ask you: I say, 'Hey, if I give you a dollar, two dollars, could you fill up this survey for me, just do it on this app', and you say, 'Fine'.
"I don't just capture what your responses are, I capture all of the information about you from Facebook, but also this app then crawls through your social network and captures all that data also.
"By you filling out my survey, I capture 300 records on average.
"And so that means that, all of a sudden, I only need to engage 50,000, 70,000, 100,000 people to get a really big data set really quickly, and it's scaled really quickly.
"We were able to get upwards of 50 million-plus Facebook records in the span of a couple of months."
He added that "almost none" of the individuals knew about how their data was used.
What has Cambridge Analytica said?
The firm said in response to the claims that it was "quite obvious" the former employee "had a grudge to bear" and dismissed his accusations as "pure fantasy".
In a response to its suspension from Facebook, Cambridge Analytica said it fully complied with Facebook's terms of services.
It added: "Cambridge Analytica's Commercial and Political divisions use social media platforms for outward marketing, delivering data-led and creative content to targeted audiences. They do not use or hold data from Facebook profiles."
The statement continued: "No data from GSR was used by Cambridge Analytica as part of the services it provided to the Donald Trump 2016 presidential campaign.
"Cambridge Analytica only receives and uses data that has been obtained legally and fairly. Our robust data protection policies comply with US, international, European Union, and national regulations."
What action is being taken?
The Information Commissioner, Elizabeth Denham, said she would investigate the circumstances "in which Facebook data may have been illegally acquired and used".
It will form part of an ongoing inquiry into the "use of data analytics for political purposes".
She added in a statement: "It is important that the public are fully aware of how information is used and shared in modern political campaigns and the potential impact on their privacy.
"We are continuing to invoke all of our powers and are pursuing a number of live lines of inquiry. Any criminal and civil enforcement actions arising from the investigation will be pursued vigorously".
Facebook said Dr Kogan, Cambridge Analytica, parent company Strategic Communication Laboratories and Mr Wylie's accounts would all be suspended "pending further information".