Billionaire investor Carl Icahn has resigned as a director at Yahoo, saying that the board no longer needs an activist among its ranks.

In a resignation letter to the board, Icahn wrote: "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.

"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 fulfil his fiduciary duties to the shareholders."

Icahn, who was an outspoken opponent of Yahoo founder Jerry Yang, upped his share in the internet giant from five to 5.4 per cent after Yang stepped down last year.

He was also instrumental in trying to force through a deal with Microsoft, after Redmond's proposed $4.7bn takeover of Yahoo fell through.

However, Icahn took a more backseat role when Yahoo finally signed a search and advertising deal with Microsoft earlier this year, selling more than 12 million shares in the company.

He was also a vocal supporter of new chief executive Carol Bartz, who stepped up at a difficult time for the company in January, and has since done well to turn things around.

"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," Icahn wrote in his resignation letter.