WASHINGTON — The $3.1 billion merger between Web search king Google and online ad giant DoubleClick approved by U.S. regulators yesterday may create an advertising powerhouse of unrivaled reach and knowledge of Internet users' lives, desires and interests.

Combined with Google's search function and the data it collects from people as they use the Internet, the acquisition of DoubleClick could result in Web surfers seeing more advertising that corresponds to their online activities. The trade-off, some say, is that users would lose control over more of their private data to Google.

Given its global scope, the deal still requires approval by the European Union, which has been stricter than the U.S. on antitrust reviews. The ambitions of big U.S. companies such as Microsoft have been curbed by European regulators.

Privacy advocates and Google's rivals have shifted their lobbying to Europe, where Google has a larger share of the online-search market.

The deal is one of many big-money mergers between Internet companies and advertising firms this year and heralds the era of data-based advertising, as companies seek more ways to acquire data about what people are doing on the Internet and deliver highly targeted advertising to them. The fear is that the collection of so much personal information by one company could expose it to theft and abuse via the Internet or even cell phones.

Google has excelled at "contextual advertising," or sending text-based ads to Google users that relate to their search topics. For example, if a user searches for a specific automobile, Google will send car ads with search results.

DoubleClick is the Internet leader for producing display advertising for Web sites — banner ads, video and such. The company has provided display advertising for almost every major online publisher, including Sports Illustrated, www.About.com and www.PriceGrabber.com.

Combined Google and DoubleClick technology could evolve so it delivers display and video advertising with sniper-like accuracy to the customers most likely to buy their advertisers' products.

Online is the fastest-growing sector in advertising, expanding at an annual rate of about 20 percent, far outpacing TV, radio or outdoor. Because the Internet can tell advertisers a great deal about who sees their ads — where they live, how much time they spend looking at the ad, which site they saw it on — companies such as Google are buying companies with the most sophisticated tools to deliver ads to Internet users.

The deal was approved without conditions in a 4-1 vote yesterday by the Federal Trade Commission. The FTC said it does not believe it is approving a Web advertising monopoly, though one commissioner and some others disagreed.

In a written statement, the commission said the merged companies would not control the user-data market, pointing out that Google's chief rivals — Microsoft, Yahoo and Time Warner — "have at their disposal valuable stores of data not available to Google."

More people use Google than any other search engine; 65 percent of all Internet searches are conducted through Google. In the third quarter of this year, the company reported revenue of $4.2 billion. Google shares closed at $689.69 yesterday, up $12.32.

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