Google has been flagged as a rival to Microsoft in a $2 billion (£1bn) race to buy DoubleClick, the largest broker of display advertising on the internet.

Yahoo! and AOL, the online division of Time Warner, have also held talks with DoubleClick, which is majority-owned by Hellman & Friedman, the San Francisco-based private equity firm.

Last week, The Wall Street Journal reported that DoubleClick had appointed Morgan Stanley to explore a possible sale of the business. DoubleClick is also thought to be pondering a return to the stock market.

Hellman & Friedman took DoubleClick private in 2005, in a deal worth about $1 billion.

DoubleClick's main business is to serve the advertisements that appear on websites. In contrast to Google’s main business, which is focused on text ads, charged for on a cost-per-click basis, DoubleClick charges for ads on a CPM (cost per thousand) basis and concentrates on display ads.

The scale of Google’s ambitions in advertising are known to extend beyond its massively lucrative search-advertising business.

A recent job advert paid for by the company sought a “head of national TV sales [responsible for building] a world-class national TV advertising sales team”.

Analysts have said that Microsoft, which trails Google in text advertising, cannot afford to allow its rival to gain a similar grip on the display market.

However, executives at the software giant have poured scorn on the valuations circulating on the web, which have suggested that Hellman & Friedman is seeking a price of about $2 billion for DoubleClick.

Don Dodge, the director of business development for Microsoft's Emerging Business Team, said on his blog: "Hellman & Friedman acquired DoubleClick a little over a year ago for $1.1 billion. Since then …they have divested two divisions of the company for $525 million, leaving a net investment of about $600 million. And they want to sell it for $2 billion?"

He added: “The revenue multiples don't make sense either. DoubleClick had about $150 million in revenue last year, with about $100 million coming from ad placement. Presumably the rest of the revenue came from businesses that were divested.

“So, H&F wants 20 times revenues for DoubleClick? Maybe 20 times earnings would make sense, but 20 times revenues? You have got to be kidding.”

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