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”.