Yahoo! Inc. agreed to let Google Inc. sell some of the advertisements it runs alongside Internet searches, seeking to shore up sales after ending talks about a combination with Microsoft Corp.

The deal may add $800 million to sales a year, Sunnyvale, Calif.-based Yahoo said in a statement yesterday. The companies will delay implementing the program for up to three and a half months to give the U.S. Justice Department time for review.

Yahoo shares sank 10% yesterday after the company said talks with Microsoft failed. The agreement with Google, which may last as long as 10 years, is part of a plan that the chief executive officer of Yahoo, Jerry Yang, has developed to fight off billionaire Carl Icahn’s proxy battle. Shareholders including Gamco Investors Inc.’s Larry Haverty already have questioned the value of a Google partnership.

RELATED: Microsoft Bid for Yahoo Probed by SEC.

“Abdicating search to Google puts Yahoo in an untenable strategic position in order to obtain short-term gains,” a spokeswoman from Soleil Securities Group Inc., Laura Martin, said in an interview. The Los Angeles-based analyst advises investors to hold Yahoo shares.

The agreement with Google may add as much as $450 million in operating cash flow in the first 12 months, the companies said. The agreement is not exclusive, meaning that other companies may be able to show ads on Yahoo.

Microsoft isn’t interested in buying all of Yahoo, even at the $33-a-share price it wanted before the end of earlier discussions May 3, Yahoo said yesterday in a statement. Microsoft proposed buying just Yahoo’s Internet search business, and Yahoo directors declined. Microsoft offered $35 a share for 16% of Yahoo as part of an alternative agreement, the Wall Street Journal reported, citing unnamed people.