Microsoft is reportedly mulling a purchase of a significant equity stake in Yahoo, which Redmond previously tried, and failed, to acquire in a rancorous and highly public takeover battle.

Microsoft, which struck a search partnership with the online portal after failing to negotiate a buyout, has signed a non-disclosure agreement that allows it to scrutinize Yahoo's financial books in contemplation of acquiring a chunk of the company, according to several published reports.

Microsoft is sizing up Yahoo in concert with a number of private equity players, including Silver Lake and TPG Capital, possibly with an eye to forming a consortium that could bid for all, or parts, of the company, according to reports by The New York Times, Bloomberg, and Reuters.

[ What other areas is Microsoft exploring? Read Microsoft Buys Video Search Engine For $100 Million. ]

Microsoft has not commented, and in the past has dismissed such stories as "speculation."

Microsoft in 2008 offered $33 per share for Yahoo, whose stock now trades in the $15 range. Yahoo insisted on $37 per share, and acquisition talks degenerated into a public spat between Yahoo co-founder and former CEO Jerry Yang, who opposed the sale, and Microsoft CEO Steve Ballmer and Yahoo shareholder Carl Icahn, who favored a deal.

Icahn publicly called Yang and Yahoo's board "irresponsible" for rejecting Microsoft's offer. In response, Yahoo's board said Icahn held "a significant misunderstanding" of the proposed deal and accused the corporate raider of trying to take control of Yahoo "for the express purpose of trying to force a sale of Yahoo."

Yang stepped down from Yahoo's top spot in November of 2008 and was replaced by Carol Bartz. Her own rocky tenure at Yahoo lasted until early September of this year, when she was effectively fired by Yahoo's board after failing to engineer a turnaround. She was replaced by CFO Timothy Morse.

In a strikingly candid interview with Forbes, Bartz said she was "f….ed over" by a Yahoo board that was trying to find a scapegoat for the company's failure to match Google in key markets like search and cloud services.

Under Bartz, Yahoo last year completed a deal with Microsoft under which Yahoo redirects search queries on its sites to Microsoft's Bing search engine. But Microsoft and Yahoo's combined share of the U.S. search market remains less than half of Google's, according to the latest data from ComScore.

As of October, Google held a stake of 65.6%, while Yahoo's share came in at 15.2% and Microsoft's stake stood at 14.8%, according to ComScore. Yahoo shares were up 1.79%, to $15.37, in afternoon trading Monday. Shares of Microsoft were up 1.89%, to $24.76.

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