UPDATE 1-EU confirms in-depth Google/DoubleClick probe

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BRUSSELS, Nov 13 (Reuters) - The European Commission confirmed on Tuesday it was opening a four-month, in-depth review of Google's (GOOG.O: Quote, Profile, Research, Stock Buzz) plans to buy rival DoubleClick for $3.1 billion.

"The Commission's initial market investigation indicated that the proposed merger would raise competition concerns in the markets for intermediation and ad serving in online advertising," the European Union's top competition regulator said in a statement, confirming a Reuters report.

The Commission has 90 working days, until April 2, to take a final decision on whether the proposed transaction would significantly impede effective competition.

"The decision to open an in-depth inquiry does not prejudge the final result of the investigation," it said.

Google, which stores data on the Internet-surfing habits of consumers, wants to buy DoubleClick to increase its clout in tailoring advertisements to consumer activities.

Both companies are involved in the sale of on-line ads, although their business models differ.

Google has already proposed alterations, and the deadline had been extended to Nov. 13 so the changes could be vetted by customers and competitors.

Google competition counsel Julia Holtz has said that in response to third-party concerns the company had committed to the Commission that it would keep certain DoubleClick business practices unchanged.

Critics have also raised questions about what effect the deal might have on privacy, but the Commission has said privacy by itself is not part of a competition review.

Google has by far the strongest position in Web searching in Europe. The acquisition has drawn vehement opposition from competitors such as Microsoft Corp (MSFT.O: Quote, Profile, Research, Stock Buzz) and Yahoo Inc. (YHOO.O: Quote, Profile, Research, Stock Buzz)

The European Commission is working closely with the U.S. Federal Trade Commission, which has been reviewing the case since May.

In the United States, there has been one congressional hearing on the deal and Republicans are pressuring for more.

Google's purchase is part of a rapid consolidation in the Internet ad industry that includes Microsoft's $6 billion acquisition of aQuantive Inc, home to the largest interactive ad agency.

Yahoo bought BlueLithium for $300 million and Time Warner Inc's AOL unit bought Tacoda.

Both of the acquired companies use cookie technology to record Web surfing habits of consumers so advertisers can target ads based on the information. (Reporting by David Lawsky and Dale Hudson, editing by Erica Billingham)