Sullivan & Cromwell has been drafted in to advise Microsoft on its landmark $44.6bn (£22.bn) attempted takeover of internet giant Yahoo after New York rival Simpson Thacher & Bartlett was conflicted out.

Simpson Thacher this week confirmed that it had been forced to withdraw from the mandate due to a conflict which could not be resolved. The firm had initially been handed the lead role for Microsoft, as reported by legalweek.com on 4 February.

The conflict means Sullivan is now fielding a team led by corporate partner James Morphy and also featuring fellow corporate partners Duncan McCurrach and Alexandra Korry in New York and Eric Krautheimer in Los Angeles.

Pete Ruegger, chairman of Simpson Thacher's executive committee said in a statement: "We hold Microsoft and its team in the highest regard. However, in order for us to fulfill our ethical obligations to each of our clients, it became necessary for us to withdraw from this representation."

Cadwalader Wickersham & Taft will continue to advise Microsoft on antitrust aspects of the approach. The firm's team is being led by global antitrust chief Rick Rule, assisted by antitrust partner Jonathan Kanter and senior associate Andy Forman.

Skadden Arps Slate Meagher & Flom landed the lead advisory role for Yahoo on the proposed deal, led by Palo Alto corporate partner Kenton King.

Microsoft's initial appointment of Simpson Thatcher was interpreted by some as a snub to Sullivan, which has historically been as one of Microsoft's main corporate advisers.

Commenting on the appointment, McCurrah said: "I don't know why we were not instructed to start with. We are glad to get it but we don't expect to have a monopoly on any client."

Microsoft announced on 1 February that it had proposed to the Yahoo board of directors to buy all outstanding shares of Yahoo's common stock as it bids to compete with undisputed online advertising market-leader Google.

The Yahoo board has yet to accept the Microsoft offer, prompting speculation among market analysts that the bid could turn hostile.

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