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 $47.5 billion (£24.4 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 last few weeks and has nominated 10 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, 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, 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 $47.5 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 Yahoo! share price the day before Microsoft's initial approach on January 31 was made public. Jerry Yang, the co-founder of Yahoo! and its chief executive, 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, their bigger rival.

In the letter to Mr Bostock, Mr Icahn claimed that: "Yahoo [sic] 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 is not clear whether Mr Icahn has been in contact with Microsoft about discussing his tactics or whether Microsoft has indicated to him whether their offer is still on the table.

It is also not clear whether Mr Yang and Mr Bostock have agreed to meet Mr Icahn to discuss his demands or whether they are continuing to pursue an alternative deal with another technology company.

After Microsoft's initial approach to Yahoo! at the end of January, Mr Yang rapidly initiated discussions with News Corporation, the owner of The Times, with AOL - the internet arm of Time Warner - and with Google, the biggest internet company in the world. It is understood that Mr Yang was seeking to secure an alternative deal that would block Microsoft from acquiring his company.

On July 3, Yahoo! shareholders will meet for the company's annual general meeting in California. At the meeting, shareholders will be asked to vote on the re-election of existing directors and also to vote on whether they want to appoint the 10 executives nominated by Mr Icahn.

Wall Street was sceptical about the chances of Mr Icahn's demands being successful. Shares in Yahoo! rose just 30 cents to $27.4, well below Microsoft's last offer of $33.

In the letter, Mr Icahn accused the Yahoo! board of having "completely botched" a merger with Microsoft and urged Mr Bostock to allow shareholders to be allowed to vote on any "strategic alternative" which could threaten a deal with the computer company.

Besides himself, Mr Icahn’s recommended board of directors includes Internet entrepreneur Mark Cuban, who became a billionaire by selling Broadcast.com to Yahoo during the dot-com boom and Adam Dell, the venture capitalist, whose brother, Michael, founded Dell and Frank Biondi, the former chief executive of Viacom.

Yahoo! has suffered sliding market share over the last 18 months, losing out to Google in the $40 billion a year online advertising market. Microsoft had wanted to buy Yahoo! so that it could close the gap between itself and Google, which is the dominant player in the market.

Yahoo! failed to return calls and Microsoft declined to comment.

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