Vendor Patrick Porter works on a laptop marked with the logo for Bing, Microsoft’s recently upgraded search engine, in a cafeteria at Microsoft in Redmond, Wash., on Wednesday. The extended reach Microsoft Corp. is gaining with its new partnership with Yahoo Inc. will let it introduce Bing to more people.

San Francisco ? Microsoft finally persuaded Yahoo to surrender control of the Internet’s second most popular search engine and join it in a daunting battle — taking on the overwhelming dominance of Google in the online advertising market.

A 10-year deal announced Wednesday gives Microsoft its best shot yet to show its new search technology, Bing, is as good as or better than Google’s. Microsoft also hopes to use Yahoo to divert sales from Google, which generates more than $20 billion a year from ads.

Gaining access to Yahoo’s audience would instantly more than triple Bing’s U.S. market share to 28 percent. That’s still a far cry from the remarkable 65 percent of U.S. searches handled by Google, according to the research firm comScore Inc.

By joining forces, Microsoft and Yahoo are betting they will be able to focus on their respective strengths. By turning over responsibility for search technology to Microsoft, Yahoo can concentrate on sales of billboard-style advertising on the Web and figuring out how to keep luring traffic to its Web sites, which already attract more than 570 million people worldwide every month.

Yahoo has been struggling so badly since then that Microsoft isn’t paying any money in advance. Instead, it will give Yahoo 88 percent of the search ad sales made on its Web site, above the usual commission of 70 to 80 percent.

By spending less on its own search technology, Yahoo expects to boost its annual operating profit by about $500 million — but not until 2012, when the two companies expect to have all the pieces of a complex technological puzzle in place.

Yahoo shares plunged $2.08, or 12 percent, as investors expressed disappointment over the absence of an immediate windfall. Microsoft shares gained 33 cents to $23.80, while Google shares shed $3.61 to $436.24.

It took Carol Bartz, Yahoo’s chief executive, just six months to strike a deal with Microsoft — something that neither of her predecessors, seemed interested in doing.

Yahoo will have limited access to the data on users’ searches, which yield insights that can be used to pick out ads more likely to pique a person’s interest.

Microsoft CEO Steve Ballmer could barely contain his excitement as he gushed about finally getting Yahoo on his side — something he has been trying to do for at least three years.

“I am very enthusiastic,” Ballmer said in an interview. “This is what I have basically been saying for the past 18 months: The world will be better served for consumers, advertisers and publishers, and there will be more competition for Google, if we can somehow figure out how to get Microsoft and Yahoo together in search.”

Antitrust regulators plan to review the agreement to make sure it doesn’t lessen competition or compromise the privacy of search engine users. Google tried to stop Yahoo from falling into Microsoft’s camp. Last year it formed its own proposed search advertising deal with Yahoo, only to be forced to retreat after U.S. antitrust officials threatened to sue.

Advertisers will probably support Microsoft because a stronger player in the search market should protect them from potential Google abuses, said Kevin Lee, CEO of online marketing specialist Didit Inc.

While Microsoft and Yahoo await government approval of their partnership, there is no doubt Google will try to increase its lead by upgrading its own search engine, said Danny Sullivan, editor of the online newsletter SearchEngineLand.

“Google is very paranoid about Microsoft,” Sullivan said. “They are always trying to figure out what kind of ‘evil’ thing Microsoft is going to do next.”