The United States' Justice Department has brought in the big guns as it probes Yahoo's deal with Google over search advertising – with antitrust expert Sandy Litvack reportedly hired as a consultant.

Litvack – a former vice chairman of Walt Disney Co – was antitrust chief at the Justice Department under Jimmy Carter's presidency, and Dow Jones believes his opinion will go a long way to deciding if there is a case to pursue.

Yahoo has already insisted that the deal with Google is legal and that any probe will underline this.

Informed

"We have been informed that the Justice Department, as they sometimes do, is seeking advice from an outside consultant, but that we should read nothing into that fact," Yahoo said in a statement to Reuters.

Both Google and Yahoo will be waiting with baited breath to see if the decision goes their way or if they will be facing a potentially expensive antitrust battle.