A deal between Google and AOL would be "disastrous" if it means the latter won't be able to seek mergers or business deals with other companies, billionaire investor Carl Icahn said in an open letter to the board of directors of Time Warner.

It's the latest in a string of open criticisms of Time Warner's board by America's 24th richest man. Icahn is attempting to rally shareholders to vote to replace a majority of the company's board of directors.

"I am deeply concerned that the Time Warner board may be on the verge of making a disastrous decision concerning an agreement with Google," Icahn said. "I believe there are and will be major opportunities to enhance Time Warner's value in future combinations. However these transactions might not be achievable if Time Warner enters into long-term arrangements that preclude future flexibility such as an agreement regarding search functionality. I also question whether Google is the best partner for unlocking the value of the AOL asset."

The comments come after reports that Google is poised to pay $1 billion for a 5 per cent stake in AOL, and similar reports that have indicated Time Warner has already ended talks on any kind of tie-up between AOL and other suitors, such as Microsoft and Yahoo.

Icahn warned Time Warner's board that shareholders would hold them personally responsible if they fail to maximise AOL's value, and reminded them of the upcoming proxy fight for control over the board. Earlier this year, Icahn and a group of investors that control just three per cent of the outstanding shares in Time Warner vowed to fight a proxy battle to gain enough shareholder votes to replace a majority of the company's board of directors.