Carl Icahn has launched a savage attack on Amylin Pharmaceuticals’ chairman Joseph Cook and called him to resign in the latest battle for control of what the billionaire investor calls a “flailing” company.

Mr Cook is under fire from Icahn Capital and Eastbourne Capital Management who together own more than 20% of Amylin’s shares and have each nominated separate slates of five directors each to sit on Amgen’s 12-man board. The company has put forward its own set of nominees.

Amylin co-founder Howard ‘Ted’ Greene and an ex-chief executive of the firm, Ginger Graham, have resigned from the board, but Mr Icahn says that himself, Eastbourne and Mr Greene “all agree that the obvious choice for the first director to leave Amylin's board is you”. His letter goes on to say that Mr Cook has been the chairman and/or CEO “during the time that an enormous amount of stockholder value has been destroyed” and “a flawed strategy has existed for far too long”.

Mr Icahn’s letter goes on to say that “like an ‘imperial’ chairman, you have taken steps to entrench yourself that we believe to be unconscionable”. That is referring to a “poison put’ which means that replacing the majority of directors on the board would trigger a provision with bondholders, allowing them to seek repayment of more than $900 million in debt. Only the current Amylin board can change a majority of directors without triggering that provision.

There is also a ‘poison pill’ which prevents shareholders acting together to push for a change of control. Mr Icahn said that the provisions mean that he may not have “meaningful conversations with other large stockholders”, before adding that “even in dictatorships dissidents are allowed to communicate with each other”.

The letter goes on to refer to Mr Icahn’s time at ImClone Systems from October 2006 through to its sale to Eli Lilly in late 2008. Its value increased by 135% and Mr Icahn says “it is interesting to note that, in direct contrast, during the same period Amylin's per share market value declined in value by 85%”.

He goes on to say that “given that Amylin is flailing, as is evident from the market for its shares, it is ludicrous and arrogant for you to contend that my highly qualified nominees, who were among those responsible for such wonderful results at ImClone, cannot be beneficial to Amylin”.

Mr Icahn adds that “with all due respect, Amylin, under your stewardship, has made many grievous mistakes”. In particular, he says that in relation to the company’s diabetes drug Byetta (exenatide), partnered with Eli Lilly, Amylin “never should have undertaken to spend enormous amounts of money to maintain a large commercial operation…an absurd commercial effort for a small biotech company”. Mr Icahn says that “had we been on the board of Amylin when its deal with Lilly was consummated, we would have fought for a royalty paying arrangement with an option to co-promote Byetta”.

Amylin has yet to repond to Mr Icahn’s letter but a statement issued earlier this month by “lead independent director” James Wilson said: “What are Icahn and Eastbourne trying to achieve with their actions? The fact is that any reasonable observer would have to conclude, based on their actions, that they appear to be seeking control of the board”.

Mr Icahn’s letter comes a week after he filed proxy materials seeking support for his four nominees to the board of directors of another US company Biogen Idec. The company, which won a previous proxy battle against Mr Icahn, has recommended that its shareholders vote against the latter’s nominees.

In January 2008, Mr Icahn criticised the management at Biogen for their inability to find a buyer for the firm.