Frances Haugen (Facebook whistle-blower) to testify before Parliament Monday

British policymakers will hear testimony on Monday from the former Facebook manager who became a whistle-blower and has shared scores of internal documents with policymakers and journalists to help build a case for stiffer oversight of the social media giant.

Frances Haugen, the former employee, is scheduled to testify before Parliament as part of her tightly choreographed campaign to reveal internal Facebook research and discussions that paint a portrait of a company vividly aware of its harmful effects on society, contrary to public statements by company leaders.

Even for Facebook, a company that has lurched between controversies since Mark Zuckerberg started it as a Harvard undergrad in 2004, Ms. Haugen’s disclosures have created a backlash and public relations crisis that stands apart. It has put the company, with more than 2.7 billion users, on the defensive, helping attract political support for new regulation in the United States and Europe and leading to some calls for Mr. Zuckerberg to step aside as Facebook’s chief executive.

The testimony in Britain on Monday is part of the next phase of Ms. Haugen’s campaign against Facebook, a company that she says has put “profit over people.” After anonymously leaking internal Facebook research to The Wall Street Journal that resulted in a series of articles that began in September, she revealed her identify early this month for an episode on “60 Minutes” and testimony before a Senate committee.

Since then, she has shared the Facebook documents with other news organizations, including The New York Times, resulting in additional stories about Facebook’s harmful effects, including its role in spreading election misinformation in the U.S. and stoking divisions in countries such as India.

Ms. Haugen is now making a tour across Europe, home to some of the world’s most aggressive tech regulation and where governments are expected to act faster than the United States to pass new laws targeting Facebook and other tech giants. After testifying before British lawmakers, Ms. Haugen is scheduled to meet in the coming weeks with officials in Brussels, Paris and Berlin. She is also scheduled to speak at an industry conference in Lisbon.

“For all the problems Frances Haugen is trying to solve, Europe is the place to be,” said Mathias Vermeulen, the public policy director at AWO, a law firm and policy firm that is among the groups working with Ms. Haugen in the United States and Europe.

British policymakers are hearing Ms. Haugen’s testimony as they draft a law to create a new internet regulator that could impose billions of dollars worth of fines if more isn’t done to stop the spread of hate speech, misinformation, racist abuse and harmful content targeting children.

The policy ideas gained additional momentum after the murder this month of David Amess, a member of Parliament, leading to calls for the law to force social media companies to crack down on extremism.

Later this week, representatives from Facebook, Google, YouTube, Twitter and TikTok are scheduled to testify before the same British committee as will Ms. Haugen.

In Brussels, Ms. Haugen is scheduled to meet on Nov. 8 with European Union officials drafting laws that would force Facebook and other large internet platforms to disclose more about how their recommendation algorithms choose to promote certain material over others, and impose tougher antitrust rules to prevent the companies from using their dominant positions to box out smaller rivals. European policymakers are also debating a ban on targeted advertising based on a person’s data profile, which would pose a grave threat to Facebook’s multibillion-dollar advertising business.

Despite growing political support for new regulation, many questions remain about how such policies would work in practice.

Regulating Facebook is particularly complex because many of its biggest problems center on content posted by users all over the world, raising difficult questions about the regulation of speech and free expression. In Britain, the new online safety law has been criticized by some civil society groups as being overly restrictive and a threat to free speech online.

Another challenge is how to enforce the new rules, particularly at a time when many government agencies are under pressure to tighten spending.