European regulators approved Google's $3.1 billion purchase of DoubleClick, which was first proposed in April 2007, saying the deal would be ''unlikely to have harmful effects on consumers.''

Hours later, Google announced it had closed the deal. Google Chief Executive Eric Schmidt said in a statement the company was ''thrilled'' to have gotten control of the leading software for providing display advertising to online publishers.

The online advertising market is generally viewed as being split between search advertising, which are ads triggered when someone types a query into a search engine, and display advertising, which are ads on Web pages that often appear as graphics or videos.

Google dominates the search advertising business, but has less than 1 percent of the display market.

But there's huge potential in the latter. According to J.P. Morgan, display advertising is expected to grow from $17 billion worldwide in 2007 to $28.6 billion in 2010.

Yahoo commands the greatest market share by advertising revenue, followed by Microsoft, AOL, MySpace and the Weather Channel, according to industry estimates.

Analysts and industry insiders said the purchase of DoubleClick could enable Google to quickly close the gap with its rivals.

''For competitors this is trouble in Dodge City,'' said Jonathan Shapiro, chief executive of MediaWhiz Holdings and a former chief strategy officer for DoubleClick.

Microsoft, AT&T and Yahoo had asked European regulators to block the deal because they said it would lead to higher prices for advertisers and publishers.

In September, Brad Smith, Microsoft's general counsel, testified before a U.S. Senate subcommittee that the deal would give Google control over 80 percent of display ads on third-party Web sites.