Billionaire investor Carl Icahn is reportedly mulling a proxy fight against Yahoo's current board members in order to pressure the company to re-establish merger negotiations with Microsoft.

Icahn has bought as many as 50 million Yahoo shares since Microsoft walked away from the deal, according to reports Tuesday in The Wall Street Journal and on CNBC, which both quoted anonymous sources familiar with the situation.

[ For the complete saga of Microsoft's unsuccessful bid to take over Yahoo, check out InfoWorld's special report ]

However, Microsoft hasn't indicated to Icahn that it would return to the negotiating table, and Icahn hasn't decided whether he will go through with the proxy fight, the reports said.

Yahoo investors have until Thursday to nominate candidates to the board, whose 10 incumbent directors are all up for re-election this year when Yahoo holds its shareholders' meeting in July. Separately from Icahn's plans, other large investors are considering getting into the fray, the Journal reported.

Icahn did not immediately respond to a request for comment, and Microsoft and Yahoo declined to comment.

The news was first reported by CNBC in the mid-afternoon, helping to give a lift to Yahoo's stock, which closed up 5 percent to $26.56 on the Nasdaq.

Microsoft announced its $44.6 billion bid for Yahoo on Feb. 1 but walked away from the deal three months later, on May 3, saying the companies couldn't agree on a price. Microsoft's last offer was for $33 per share, or about $5 billion more than its original offer, but Yahoo wanted $37 per share.

Since then, various big Yahoo shareholders have expressed their displeasure with Yahoo's board and management for, in their view, not negotiating in good faith with Microsoft and causing the talks to collapse. Yahoo formally rejected Microsoft's original offer on Feb. 11, saying it undervalued the company.

On Monday, May 5, the first day of trading after Microsoft's offer withdrawal, Yahoo's stock lost significant value, closing down 15 percent at $24.37, after dropping as low as $22.97 during the day.

Last week, Yahoo co-founder and CEO Jerry Yang and other top Yahoo executives tried to shift the blame to Microsoft, alleging that the $33-per-share offer was never put in writing and that Microsoft unexpectedly walked away at a time when Yahoo was still open to negotiating.

At the same time, Microsoft's top brass, including Chairman Bill Gates, have repeatedly said that Microsoft has closed the book on its attempts to buy Yahoo and that it is moving on to other options, namely growing its Internet business "organically," meaning via internal efforts and not big acquisitions.

In the meantime, a much-publicized deal in which Yahoo would outsource part of its search advertising business to Google -- the possibility of which Microsoft CEO Steve Ballmer cited as a major reason to withdraw the offer -- has yet to be finalized, and, according to recent anonymously sourced press reports, has lost steam.

All along, Microsoft had indicated its readiness to launch a proxy fight to oust Yahoo's board and replace it with its own candidates, but eventually, as Ballmer explained, Microsoft decided against that option, saying that it wasn't interested in engaging in a hostile and potentially long process.

Microsoft's main goal in acquiring Yahoo had been to give its underperforming Internet business a boost and turn it into a stronger competitor against Google.

Icahn is well known for taking to task the CEOs of companies he invests in when he feels they aren't doing a good job of delivering shareholder value.