BRUSSELS, Belgium - Google Inc. said Friday it had filed for European Union permission to take over online ad tracker DoubleClick, a $3.1 billion deal that has already stirred concern about the control it will give Google over Internet advertising.

"We asked the European Commission to look at the proposed acquisition. We believe this deal is positive for both users and advertisers and fosters competition," Google's antitrust counsel Julia Holtz said in a statement.

Search engine rival Yahoo Inc. immediately said the deal raised important questions about the future of advertising on the Web.

"These questions warrant an in-depth debate and review by a broad range of Internet publishers, advertisers, service providers and governments in Europe and elsewhere," said Yahoo Europe's managing director Toby Coppel.

Microsoft Corp. -- which also had wanted to buy DoubleClick -- said its views had not changed since April when it warned that the deal "raised serious competition and privacy concerns" by giving unprecedented control over online advertising.

Using cookies and user logs, the world's largest Internet search engine compiles data on the search terms specific users enter, as well as other potentially sensitive online information.

Google says that information helps its search engine better understand its users and deliver more relevant results and advertisements.

The concern of competitors is that concentrating so much valuable historical information on users in one company's hands could make it difficult for newer companies to enter the field.

But privacy advocates say sharing the information Google collects -- which might be the ideal answer to reduce antitrust concerns -- raises the prospect of violating consumers' right to keep information about themselves secret.

Google took the unusual step of asking EU regulators to start looking at "all aspects of the transaction" two months ago.

Once the EU receives a request to clear a deal, it has just 25 working days to decide whether to wave it through or open a deeper investigation that can take up to four months.

During that time it can ask a company to make changes to the deal in order to eliminate antitrust worries. These can see companies sell off units or make binding promises to do things differently.

Google refused to comment on what it might do to win EU approval, saying it was too early.

But several technology companies, who did not want to be named because the investigation in confidential, have said in recent weeks that they received questionnaires from the European Commission asking for their views on the deal -- a highly unusual move for regulators.

The questions apparently address how the deal will affect online advertising and data privacy because of the volume of data on how people use the Web and what they search for that Google will have.

New York-based DoubleClick helps its customers place and track online advertising, including search ads, which Google -- more than its nearest search competitors Yahoo and Microsoft Corp. -- already has turned into an extremely lucrative business.

DoubleClick places ads on Web pages that targeted consumers are likely to use -- generating money for smaller publishers and lesser-visited pages. Publishers, search engines and advertisers don't know the full prices one another pay or receive for ads. DoubleClick is the messenger among them, and knowing its prices could allow a publisher or search engine -- like Google -- to increase the margin it charges each end.

The European consumer organization known as BEUC also said it was sticking by a July letter to regulators when it said the takeover could damage privacy rights and limit consumers' choice of Web content.

The organization said people who agreed to give their details to DoubleClick could not have imagined this would be transferred to Google and many Web users are unaware that companies build up profiles about them.

U.S. consumer privacy advocacy groups have asked the U.S. Federal Trade Commission to look at how the two companies, when combined, could exploit enormous amounts of personal information users produce as they click on Web pages and applications.