United Airlines Holdings Inc. notified 36,000 U.S. employees, or 45% of the workforce, that their jobs are at risk after federal payroll aid expires at the end of September.

The final layoff tally hasn’t been finalized and may be smaller as workers weigh offers to leave voluntarily, United told its workers Wednesday. The planned furloughs include about 15,000 flight attendants, 11,000 customer service staff and 5,500 maintenance employees. About 3,700 workers have already taken voluntary separation packages.

United’s warning signals the depth of potential job losses at U.S. airlines later this year, even after the federal government provided $25 billion in payroll support plus another $25 billion available in loans. The carrier told employees July 6 that state quarantines prompted by a jump in coronavirus infections were jeopardizing a nascent U.S. travel recovery. United expects that travel demand will remain weak until a treatment or vaccine is widely available.

“We are living through the most disruptive financial crisis in the history of commercial aviation,” the company said in a letter to employees. “The reality is that United simply cannot continue at our current payroll level past October 1 in an environment where travel demand is so depressed.”

The shares fell 1.7% to $31.99 at 11:29 a.m. in New York amid broad declines at U.S. airlines. United tumbled 63% this year through Tuesday, the biggest drop on a Standard & Poor’s index of major carriers.

United’s rivals are also studying major job cuts. Last week, American Airlines Group Inc. said it would have over 20,000 more employees than needed to operate a reduced flying schedule later this year. Delta Air Lines Inc. said last month it would inform almost 2,600 pilots about a possible furlough and encouraged a total of 7,900 eligible aviators to accept early retirement.