America Online on Tuesday agreed to sell a 5 percent stake to Google Inc. in a $1 billion deal that deepens the ties binding two of the Web’s most popular sites while thwarting Microsoft Corp.’s efforts to grab a larger piece of the booming Internet advertising market.

Approving the expanded alliance had been considered a mere formality since last Friday when AOL’s corporate parent, Time Warner Inc., abruptly ended several months of negotiations with Microsoft, which had hoped to supplant Google as AOL’s main advertising partner.

Many of the details, including a plan that may display more graphical ads on Google’s traditionally sparse Web pages, had been leaked to the media in the last few days. None of the so-called banner ads will appear on Google’s home page or alongside its primary search results.

There was one significant new twist in Tuesday’s official announcement: users of AOL’s Internet-leading instant messaging service will be able to communicate with the users of Google’s 4-month-old service.