Decades of hard-fought labor concessions at United Airlines could go out the window Thursday when the

UAL Corp.

(UAL) - Get Report

unit makes a normally routine filing in its chapter 11 bankruptcy proceeding.

The airline, which sought chapter 11 protection earlier this month, will file a so-called 1113 motion in bankruptcy court Thursday, according to the

Washington Post.

The filing sets the stage for new labor negotiations and could sound the death knell for collective-bargaining agreements that have been in place for decades.

United was forced into bankruptcy proceedings when it failed to secure a billion-dollar loan guarantee from the federal government three weeks ago. It claims to be losing up to $20 million a day as it tries to reorganize and is reportedly trying to cut about $2.4 billion from its expense structure through the auspices of the bankruptcy court.

A union representing about 20,000 United flight attendants told members in a memo that it plans to negotiate "in good faith to provide United the savings needed for a successful reorganization," though it will seek changes least damaging to its members, the newspaper reported.

"The company has indicated that nothing in its presentation is 'take it or leave it,' " and that 'there is room for negotiations,' " the memo said, but the group called the airline's lengthy list of potential cuts "overreaching and unrealistic."