The deal represents the largest acquisition in Google''s history and comes just six months after Google paid $1.65 billion to acquire video-sharing site YouTube.

The DoubleClick acquisition will strengthen Google''s, search-based advertising business on the internet, as it expands into print, radio, video, mobile and TV ad markets.

The combination will also boost the ad targeting and analysis capabilities that Google can offer advertising customers.

DoubleClick, a leading independent player in the first generation of online advertising during the 1990s, has been majority-owned by San Francisco private equity firm Hellman & Friedman since 2005.

Hellman & Friedman paid $1.1 billion in stock and debt for its stake. JMI Management is a co-investor in the company.

Microsoft Corp. also involved in bidding for DoubleClick, is said to have withdrawn after the auction price rose above $2 billion. Yahoo Inc. and Time Warner Inc.''s AOL online unit were also interested in DoubleClick.