It looks like Facebook knows who's behind a shadowy £257,000 ad campaign to torpedo Theresa May's Brexit deal - but it's keeping it under wraps

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British Prime Minister Theresa May.

• Facebook is under renewed pressure to reveal who is behind a shadowy campaign to torpedo UK Prime Minister Theresa May's Brexit deal.
• In October 2018, British politicians uncovered a £257,000 ad campaign from the anonymous Mainstream Network asking people to write to their MPs asking them to vote against May's Brexit deal.
• Facebook has now written to the UK's Information Commissioner's Office with more details, according to an influential lawmaker, but the information remains confidential.

LONDON - It looks like Facebook has uncovered who was behind a shadowy £257,000 advertising campaign lobbying to kill UK Prime Minister Theresa May's Brexit deal with the EU.

The Digital, Culture, Media, and Sport Committee (DCMSC) - a parliamentary committee that last month published a damning report on Facebook privacy and disinformation - uncovered evidence of the dark ad campaign in October last year.

It found that a mysterious organisation called Mainstream Network spent £257,000 ($335,148) on ads asking users to write to their MPs calling on them to vote against May's Brexit deal. The committee found that the ads had reached up to 11 million people over a 10-month period.

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Conservative MP Damian Collins, who heads up the DCMSC, said in October that the Mainstream Network was a "clearly sophisticated organisation" and that the committee had "absolutely no idea who is behind it." At the time, he added: "The only people who know who is paying for these adverts is Facebook."

Now it seems Facebook has figured it out.

Speaking at a London event spotlighting Facebook and democracy on Tuesday, Collins said he still doesn't know who is behind the campaign, but added that Facebook sent a letter to UK privacy watchdog, the Information Commissioner's Office (ICO), about Mainstream Network on Monday.

"We still don't know who's running those campaigns, at least we don't have that information publicly. Although I understand the information commissioner has now received that information from Facebook in a letter yesterday from Facebook saying that," he said. "That's been released confidentially, Facebook told me."

Business Insider contacted the ICO and Facebook to ask for confirmation of the letter, and whether either party could disclose any further details. They did not immediately respond to the request for comment.