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.