She gave the FTC until Friday to seek an order in the San Francisco-based 9th U.S.
The FTC had no immediate comment on whether it would appeal and what arguments it might make before a three-judge panel.
"The FTC may have difficulty on appeal establishing that fact - without which the case as they framed it goes away," Crane said.
In her 53-page order, Corley said it was not enough for the FTC to argue that "a merger might lessen competition - the FTC must show the merger will probably substantially lessen competition."
A trial before an administrative law judge at the FTC begins on Aug. 2.
Persons:
Jacqueline Scott Corley, Corley, Daniel Crane, Crane, Luke Hasskamp, Robert Lande, Joseph Alioto, Alioto, Mike Scarcella, Matthew Lewis
Organizations:
U.S . Federal Trade Commission, Microsoft, Activision, Circuit, Appeals, FTC, University of Michigan Law School, University of Baltimore, Thomson
Locations:
U.S, San Francisco, Washington