Carl Icahn, the billionaire activist investor, has launched a boardroom coup against Yahoo!, threatening to oust its founder and seeking to force the online search engine to accept a $47 billion (£24.1 billion) offer from Microsoft.

Mr Icahn, who has built a career taking stakes in companies whose share price is weak, and threatening them to compel a change of strategy, has spent $1 billion acquiring a 4 per cent stake in Yahoo! in the past few weeks and has nominated ten directors to the internet company’s board.

He is also seeking regulatory permission to more than double his stake to boost his chances of forcing Yahoo! to yield to his demands.

In a letter to Roy Bostock, the chairman of Yahoo!, Mr Icahn indicated that he had secured the support of “a number” of other shareholders who had asked him to lead a proxy fight to try to “remove the current board . . . and attempt to negotiate a successful merger with Microsoft”.

It is not known the size of the combined shareholding that Mr Icahn claims to represent.

Yahoo! this month rejected an approach by Microsoft that valued the search engine at $47 billion, and which would have seen investors receive $33 a share paid in cash and stock in the software giant. The offer represented a 72 per cent premium to the Yahoo! share price the day before Microsoft’s initial approach on January 31 was made public.

Jerry Yang, the co-founder and chief executive of Yahoo!, had told Microsoft that the offer was too low and indicated that should the computer company launch a formal hostile takeover, he would hive off lucrative parts of his business to Google, its bigger rival.

In the letter to Mr Bostock, Mr Icahn wrote: “Yahoo! has acted irrationally and lost the faith of shareholders and Microsoft. It is quite obvious that Microsoft’s bid of $33 per share is a superior alternative to Yahoo’s prospects on a standalone basis. I am perplexed by the board’s actions.”

He attacked the Yahoo! board for failing to allow shareholders to vote on the Microsoft offer, a move which he described as “unconscionable”.

It also emerged yesterday that Paulson, the hedge fund that took trading positions that effectively bet on the sub-prime crisis, has built up a stake of about 50 million shares in Yahoo!.

The holding is equivalent to a 4 per cent stake. It is not known whether the fund is actively supporting Mr Icahn.

— CBS, the US television network, said that it would buy CNet Networks, the web media company, for about $1.8 billion, to boost its internet reach. CNet.com has technology reviews and news, but the firm also owns TV.com and GameSpot.com. Its shares rose $3.45, or 43 per cent, to $11.40.