WASHINGTON — The Justice Department is appealing the ruling a federal judge issued last month allowing AT&T Inc. to purchase Time Warner Inc. in the biggest antitrust case this century.

Craig Conrath, one of the Justice Department lawyers who argued the case, submitted the formal notice of appeal to the U.S. Court of Appeals for the District of Columbia Circuit Thursday.

The notice was one sentence,and a Justice Department spokesman declined to comment further.

The Justice Department’s move comes after lawyers there decided not to seek an emergency stay of U.S. District Judge Richard Leon’s June 12 ruling that cleared the way for the $85.4 billion deal after a six-week trial

Leon strongly urged the Justice Department not to seek the stay pending appeal, warning it could cause the merger to collapse because of a June 21 deadline the companies had set to close.

AT&T and Time Warner closed the deal June 15.