The agreement "would have enabled Yahoo! to replace a significant portion of its own Internet search results advertisements with search results advertisements sold by Google," a statement from the Justice Department said.

A Department of Justice inquiry found the two companies "would have become collaborators rather than competitors for a significant portion of their search advertisement businesses," the statement said.

Their collaboration would have been powerful.

"Google is by far the largest provider" of Internet search advertising and Internet search syndication, commanding 70 percent of both markets, the statement said.

Add Yahoo! into the mix, "Google's most significant competitor in both markets," and you come up with market shares of 90 percent in Internet search advertising and 95 percent of Internet search syndication, the statement said.