The decision could provoke a showdown between two of the world's most prominent technology companies with Internet search leader Google Inc. looming in the background. Leery of Microsoft expanding its turf on the Internet, Google already has offered to help Yahoo avert a takeover and urged antitrust regulators to take a hard look at the proposed deal.

If the world's largest software maker wants Yahoo badly enough, Microsoft could try to override Yahoo's board by taking its offer — originally valued at $31 per share — directly to the shareholders. Pursuing that risky route probably will require Microsoft to attempt to oust Yahoo's current 10-member board.

Alternatively, Microsoft could sweeten its bid. Many analysts believe Microsoft is prepared to offer as much as $35 per share for Yahoo, which still boasts one of the Internet's largest audiences and most powerful advertising vehicles despite a prolonged slump that has hammered its stock.

Yahoo's board reached the decision after exploring a wide variety of alternatives during the past week, according to the person who spoke to The Associated Press. The person didn't want to be identified because the reasons for Yahoo's rebuff won't be officially spelled out until Monday morning.

Microsoft and Yahoo declined to comment Saturday on the decision, first reported by The Wall Street Journal on its Web site.

Yahoo's board concluded Microsoft's offer is inadequate even though the company couldn't find any other potential bidders willing to offer a higher price.

Without other suitors on the horizon, Yahoo has had little choice but to turn a cold shoulder toward Microsoft if the board hopes to fulfill its responsibility to fetch the highest price possible for the company, said technology investment banker Ken Marlin.

"You would expect Yahoo's board to reject Microsoft at first," Marlin said. "If they didn't, they would be accused of malfeasance."

But by spurning Microsoft, Yahoo risks further alienating shareholders already upset about management missteps that have led to five consecutive quarters of declining profits.

The downturn caused Yahoo's stock price to plummet by more than 40 percent, erasing about $20 billion in shareholder wealth, in the three months leading up to Microsoft's bid.