REDMOND, Wash. - Microsoft Corp. has pounced on slumping Internet icon Yahoo Inc. with an unsolicited takeover offer of $44.6 billion in its boldest bid yet to challenge Google Inc.’s dominance of the lucrative online search and advertising markets.

The surprise offer of $31 per share, made late Thursday and announced Friday, comes with Sunnyvale-based Yahoo in a vulnerable position.

MSNBC is a joint venture of Microsoft and NBC News.

In a statement Friday, Yahoo said it will “carefully and promptly” study Microsoft’s bid.

With its profits steadily sliding, Yahoo’s stock slipped to a four-year low earlier this week and a new management team has been trying to steer a turnaround but sees more turbulence through 2008.

The announcement sent Yahoo’s share price up sharply, while Google saw its shares fall, weighted down by a fourth-quarter earnings report, released Thursday, that missed Wall Street expectations.

In a letter to Yahoo’s board of directors, Microsoft Chief Executive Steve Ballmer indicated the world’s largest software maker is determined to bring the two companies together.