LONDON — Prime Minister Theresa May has struggled to build support for her plan for Britain’s exit from the European Union. Now, it turns out, some of the opposition has come from an unknown organization posting ads to millions of people on Facebook.
In the past 10 months, the organization spent more than 250,000 pounds on ads pushing for a more severe break from the European Union than Mrs. May has planned. The ads reached 10 million to 11 million people, according to a report published on Saturday by a House of Commons committee investigating the manipulation of social media in elections.
The ads, which disappeared suddenly this week, linked to websites for people to send prewritten emails to their local member of Parliament outlining their opposition to Mrs. May’s negotiations with the European Union.
“We voted to leave the E.U., to take back control of our money and borders,” one ad said.
Who was behind the campaign remains a mystery. The name attached to it was Mainstream Network, a group that does not appear to exist in Britain, beyond the ads and a website. There is no information on Facebook or on Mainstream Network’s site about who is behind the organization.
The government panel, the Digital, Culture, Media and Sport Committee, said the posts highlighted Facebook’s continuing problem monitoring political advertising on its social network.
“Here we have an example of a clearly sophisticated organization spending lots of money on a political campaign, and we have absolutely no idea who is behind it,” Damian Collins, the chairman of committee, said in a statement. “The only people who know who is paying for these adverts is Facebook.”
The panel has been investigating the role of social media in elections, including Facebook’s influence on the country’s contentious 2016 vote to leave the European Union. It is expected to release a full report in the coming weeks.
Rob Leathern, director of product management at Facebook, said the company will update its disclosure policy in Britain next month. It will require political advertisers to verify their identities and then attach accurate information about their identities to the ads.
The changes are part of new political advertising policies that Facebook announced this week for users in Britain. No only will political ads need to be more clearly labeled, but the company is establishing a searchable archive of political ads that have been published on the site.
“We know we can’t prevent election interference alone,” Mr. Leathern said, “and offering more ad transparency allows journalists, researchers and other interested parties to raise important questions.”
Britain isn’t the only country grappling with unknown financiers of political ads on Facebook. In the United States, Facebook advertisements from unknown donors have also begun appearing in congressional campaigns.
To oversee its response to a growing number of regulatory challenges around the world, the company announced on Friday that Nick Clegg, a former deputy prime minister in Britain who is politically well connected in Europe, would become its new head of global public policy.
The Mainstream Network ads were taken down after Facebook announced its new political advertising rules in Britain, said Mike Harris, the chief executive of 89up, a social media marketing company that the parliamentary committee hired to help with its investigation.
Mr. Harris, who specializes in political campaigns, discovered the ads recently when one popped up in his social media feed. His company, which has also done work for groups in favor of remaining in the European Union, found more than 70 ads posted over a 10-month period.
The group behind the ads appears to be well funded. Based on what comparable political Facebook ad campaigns cost in the country, 89up estimated Mainstream Network spent £257,000, or about $335,000. Mainstream Network also maintains a polished website that mixes commentary favoring a hard break from the European Union, alongside straighter coverage of events such as Amazon’s announcement that it will add jobs in Britain.
Mainstream Network doesn’t give any clue of who is publishing the content. No contact information is listed on its website or Facebook page.
“There is no indication of who’s behind it, or who’s backing it,” Mr. Harris said in an interview. “This could be a wealthy individual, this could be a group of volunteers that has come together that decided to hide its identity, or it could be a foreign state. It’s totally unclear.”
The ads were disclosed at a politically fragile time in Britain, where Mrs. May is trying to balance the position of those who want to retain closer ties to the Continent against those who want a harder break.
Mainstream Network’s ads have strongly targeted Mrs. May’s central negotiating position, known as the “Chequers plan,” which would maintain a tight trading relationship with Europe. Two members of her cabinet resigned over her approach.
Pro-Brexit hard-liners want her to scrap the plan and propose a more distant relationship, like the European Union’s trade agreement with Canada.
The disclosures released on Saturday are a prelude to other investigations into online misinformation scheduled to be released by the end of the year. In addition to the parliamentary committee’s final report, highly anticipated findings are expected from the Information Commissioner’s Office after its investigation of Cambridge Analytica, the London-based political targeting firm that harvested the personal data of millions of Facebook users.