AT&T's decision to focus on its antitrust battle with the U.S. Justice Department for its purchase of T-Mobile may have backfired, irritating the judge overseeing the case and laying the groundwork for a possible deal-killing delay.

The Justice Department said on Friday it would seek to stay or dismiss its lawsuit to stop the $39 billion deal, saying it was effectively dead without approval from telecommunications regulators.

It's not a real transaction until they file with the FCC, Joseph Wayland, the Justice Department's lead attorney in the case, told U.S. District Judge Ellen Huvelle during a status hearing on the case.

AT&T and Deutsche Telekom's T-Mobile moved in November to withdraw their filing with the Federal Communications Commission to focus on the antitrust battle.

Both regulators have to give their blessing to the deal, and the FCC emerged recently as a second potent source of opposition by moving to refer the merger to an internal FCC judge and by issuing a staff report savaging the deal for curbing competition and destroying jobs.

Wayland said the government planned to file next week. It was unclear when Huvelle would rule on the motion.

AT&T's attorney Mark Hansen repeatedly pressed Huvelle on the need to move on with the current trial, slated to begin in February, arguing that the FCC and the Justice Department court fight were largely parallel.

The issues in the antitrust case (at the district court) are the same as the FCC, he said, saying that if they won in court they could then refile with the FCC.

But Huvelle repeatedly expressed skepticism about AT&T's desire for a speedy resolution, citing withdrawal of the FCC application and potential changes in the deal reported in the media.

It's a bit presumptuous to say nothing has changed, Huvelle said during an extended discussion with Hansen. You could change the deal in a month... We have no confidence that we're not being spun.

Hansen argued that an extended delay would give the Justice Department effectively a veto on the deal without having to prove the merits of its antitrust case in court. We are committed to concluding this transaction, he said.

AT&T, which has already set aside $4 billion in cash and spectrum to give T-Mobile in case the deal fails, faces a final deadline of mid-September to get the transaction done.

What's clear is that this deal is hanging on by its fingernails, said Evan Stewart, an antitrust expert with Zuckerman Spaeder LLP. Will there be a trial? Hard to believe.

A former FCC official, who asked not to be named, said that AT&T's strategy, while frustrating for the judge, could well be legal.

AT&T has a reasonable argument that no law requires it to seek approvals in any particular order and that a win in the DOJ case would make FCC approval extremely likely, said this former official.

But, he added: In the end, though, this is just another potentially crippling obstacle for a deal that is pretty close to dead.

Wayne Watts, AT&T's general counsel, reiterated on Friday the company's view that the deal would increase wireless capacity and efficiencies. We are eager to present our case in court, he said in a statement.

Sprint and regional carrier C Spire Wireless have also filed private lawsuits to stop the AT&T deal.

The case is USA v. AT&T, T-Mobile USA Inc and Deutsche Telekom AG, U.S. District Court for the District of Columbia, case No. 11-1560. Also before the court are: Sprint Nextel Corp v. AT&T Inc et al, No. 11-1600 and Cellular South v. AT&T, No. 11-1690.

(Reporting by Diane Bartz with additional reporting by Jasmin Melvin in Washington and Nicola Leske in New York; Editing by Tim Dobbyn)