The EU's executive Commission set an April 2 deadline by which to reach a final decision on the deal, which has raised concerns by Google's rivals Microsoft Corp. and Yahoo Inc. - both of which fear it will shrink competition for Internet advertising.
The European Commission said its new inquiry would "investigate whether the merger, which combines the leading providers of ... online advertising space and intermediation services, and, on the other hand, ad-serving technology, could lead to anti-competitive restrictions for competitors operating in these markets and thus harm consumers."
It said a preliminary probe, launched after Google notified the EU of its bid for New York-based DoubleClick in September, found the proposed merger "would raise competition concerns."
Yahoo Europe's Managing Director Toby Coppel welcomed the EU investigation, saying it "was needed."
"Competitive online advertising markets in Europe are of great importance to publishers and advertisers, as well as being key to innovation and consumer choice," Coppel said.
Google did not immediately return a call for comment.
The EU investigation will be conducted over 90 working days, the Commission said in a statement.
Other advertisers have expressed concerns that the deal would leave Google in a dominant position on the Internet, while consumer advocates have cited concerns about data privacy and the knock-on effect on media that increasingly rely on Internet ads to pull in revenue.
Google has insisted, however, that joining it with DoubleClick would help expand the booming Internet ad market.
DoubleClick helps its customers place and track online advertising, including search ads, which Google - more than its nearest search competitors Yahoo and Microsoft - has turned into an extremely lucrative business. It places ads on Web pages that targeted consumers are likely to use, generating money for smaller publishers and lesser-visited pages.
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