"I don't believe that it is necessary at this time to have an activist on the board of Yahoo! and currently, my attention is focused on other matters," Icahn, one of the company's largest shareholders, wrote in a resignation letter to the board, a copy of which was provided by his office.

"As a result, I do not presently have the time that is necessary to devote to the business and affairs of Yahoo! required if a board member is to fulfill his fiduciary duties to the shareholders," he added.

A spokeswoman for the investor, Susan Gordon, said in a statement that Icahn "indicated that there are a number of other companies on which he is currently focused."

Icahn - a financier who has developed a reputation as a corporate raider - on Monday offered a US$6 billion loan to ailing lending giant CIT Group.

In late August Icahn sold 12.7 million shares in Yahoo!, cutting his stake in the company a month after it formed an Internet search partnership with software giant Microsoft.

Last November, following the decision of Yahoo! co-founder Jerry Yang to step down as head of the Internet firm, Icahn increased his stake in Yahoo! to 5.4 percent, up from the 5.0 percent he owned previously.

Icahn and Yang were involved in a very public dispute last year when Yang rejected a US$47 billion takeover bid by Microsoft for the company he founded with a Stanford University classmate in 1995.

Icahn headed a shareholder revolt to oust Yahoo! leaders for bungling the Microsoft deal.

Yahoo! last year avoided an ugly show-down with Icahn, whose campaign to overthrow the board was ended by a truce that assured him three seats.

"Icahn was there to drive the sale to Microsoft," said analyst Rob Enderle of Enderle Group in Silicon Valley.

"Microsoft doesn't really want to buy Yahoo! and Yahoo! isn't interested in being acquired. So, what's he going to do? He probably figures his time is better spent elsewhere."

Icahn's sale of Yahoo! stock came a month after the company agreed to a Web search and advertising partnership with Microsoft.

"Carl has been an important member of the board and has helped us through some significant transitions," Yahoo! said Friday in a release.

"We are all grateful for his active role shaping the future of Yahoo! and wish him well in all of his endeavors."

In his resignation letter he also praised Yahoo!'s new chief executive Carol Bartz, who took over after Yang's departure.

"Carol is doing a great job and I believe the Microsoft transaction will provide great long term benefits, the potential of which many still do not understand."

In 2008, Icahn's net worth was estimated to stand at US$14 billion, but this year, following the stock market slump, Forbes magazine recalculated his net worth as US$9 billion.