Because Twitter is based in the United States, it could argue that it abides by the law and that any plaintiff would need to try the case in the United States, legal analysts said. But Twitter is opening a London office, and the rules are more complicated if companies have employees or offices in foreign countries.

Still, Twitter has resisted turning over this type of information in the past. Mr. Macgillivray, when asked about international laws at a conference in March, said, “We tell them, we’re a U.S. company, we have C.D.A. 230 here, and you’re welcome to come and try your hand at suing us here,” referring to the Communications Decency Act, which says Web companies are not liable for what their users post.

There is no question that the Twitter posts about the soccer player would be considered legal in the United States, Internet law experts said, because of the Communications Decency Act and the First Amendment’s protections for anonymous speech. Further, an injunction to prevent this type of information from being exposed would be unheard of in the United States. But the issue gets murkier across borders.

“You would think at this point in the Internet you would have a clear body of law about what laws the Internet is subject to, but that is anything but clear,” Mr. Burke said.

Other American Internet companies have run up against conflicting international laws. When Google opened its search engine in China, it censored results in accordance with Chinese law, and it has faced legal issues in Germany over the use of photographs taken for its Street View maps.

Web sites where users post content grapple with different restrictions. Some companies handle the issue by opening localized sites that sometimes follow different rules. Yelp and eBay, for instance, operate different sites in different countries.

In Britain, super-injunctions have been hotly debated. Many Britons are outraged that celebrities enmeshed in scandals and businesses engaged in dubious practices have received the injunctions with little oversight.