Microsoft Corp and Yahoo Inc have launched a 10-year Web search deal to challenge market leader Google but stopped short of combining other advertising businesses or suggesting any deeper ties.

The long-expected deal means Microsoft’s new Bing search engine will be combined with Yahoo’s experience attracting advertisers in the first serious threat to Google Inc — if the companies get regulatory approval and can make the partnership work.

Yahoo shares fell 12 percent as some investors were disappointed by the limited scope of the deal, which did not include up-front payments for Yahoo. Some investors had expected up to $3 billion up-front, according to a Bernstein report.