The European Union started legal action against Britain on Tuesday for not applying EU data privacy rules that would restrict an Internet advertising tracker, called Phorm, from watching how users surf the web.

They also warned that they could force social networking sites like Facebook or MySpace to hide minors' profiles from search engines.

The European Commission said Britain should outlaw Internet traffic interception and monitoring unless users give explicit consent that their behavior can be tracked and analyzed.

It said it had received numerous complaints about BT Group PLC, which tested Phorm in 2006 and 2007 without informing customers involved in the trial. Phorm analyzes Internet users' behavior so it can target them with advertising that might appeal to them.

"Such a technology in the view of the European Commission and European data protection law can only be used with the prior consent of the user," said EU spokesman Martin Selmayr.

Regulators sent a first legal warning to Britain on Tuesday, asking it to explain or change the way it interprets EU rules, because it currently allows interception when it is unintentional or when a tracker has 'reasonable grounds' to believe that consent was given.

Britain has two months to reply. The European Commission can issue more warnings before it can take a government before an EU court, where it may be ordered to change national law or face daily fines.

BT sought consent from users when it once again tried out Phorm from October to December 2008 in an invitation-only trial. The company says on its Web site that the trial didn't keep or pass on information that could identify users and what they did. It gave no comment on Tuesday on the EU statement.

Internet companies, privacy advocates and regulators disagree on what kind of traffic data is personal -- such as IP addresses that...