Seven US states threw their support on Friday behind the Department of Justice lawsuit seeking to block AT&T's $39 billion takeover of T-Mobile.
The Justice Department said the attorney generals of California, Illinois, Massachusetts, New York, Ohio, Pennsylvania and Washington had joined the suit filed in US District Court here last month.
The move is another setback for AT&T as it seeks to salvage the proposed takeover of T-Mobile USA, the US unit of Deutsche Telekom.
"We have had an excellent working relationship with a number of state attorneys general and they have provided invaluable assistance throughout our investigation," the Justice Department said in a statement.
"We are pleased that these states have joined the department in its lawsuit," it said. "Together, we will seek to protect consumers from the anti-competitive harm that would result from this proposed transaction."
AT&T submitted a defense of its takeover bid for T-Mobile to the court last week, rejecting the Justice Department's assertion that the deal would be anti-competitive and claiming that it would benefit consumers.
"Rather than substantially reducing competition, the combined firm will usher in more intense competition to an already vibrantly competitive market," AT&T said.
US District Court Judge Ellen Huvelle has ordered lawyers for the Justice Department, AT&T and T-Mobile to attend a meeting on Wednesday to discuss the prospects for a settlement.
In the event the case goes to trial, the Justice Department proposed on Friday that it begin on March 19, 2012. AT&T has proposed a January 16, 2012 trial date.
Explore further: Judge sets first hearing in AT&T anti-trust case