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.

Read more: We ran 2 fake ads pretending to be Cambridge Analytica — and Facebook failed to catch that they were frauds

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."

When contacted for comment by Business Insider, an ICO spokeswoman said: "We received a response from Facebook to an Information Notice issued by the ICO. The information is under review and forms part of our ongoing investigation into the use of data analytics for political purposes."

Facebook did not immediately respond when contacted by Business Insider.