The $8.50 a share offer accepted by the board was below the rumored level, and DoubleClick’s stock fell 5.48% to $8.10 after the deal was announced.

DoubleClick was effectively put up for sale last November when it said it had retained bankers Lazard Freres & Co to look at strategic options for the business. Hellman was a natural buyer for the company as the San Francisco company has a history of investing in advertising outfits.

A survivor of the dot-com bubble, New York-based DoubleClick has failed to capitalize on the internet advertising boom, which has brought soaring revenue to companies such as Google and Yahoo.

In its first quarter to March 31, it plunged into the red with a net loss of $534,000 compared with income of $7.7m on revenue 12.2% higher at $76.3m.

Kevin Ryan is to step down as DoubleClick CEO once the deal closes to pursue other opportunities.