(The Hill) — U.S. airlines are warning Congress that if the government shuts down at midnight Wednesday, flights could be delayed because air traffic controllers and Transportation Security Administration employees would be required to work without pay.

Airlines for America, a trade group that represents major carriers such as American Airlines, Delta Air Lines, Southwest Airlines and United Airlines, warned that furloughing tens of thousands of federal workers would mean longer wait times at airports and delayed flights.

“When federal employees who manage air traffic, inspect aircraft and secure our nation’s aviation system are furloughed or working without pay, the entire industry and millions of Americans feel the strain,” the group said in a statement reported by Reuters.

U.S. air carriers warn that the “system may need to slow down,” which would reduce efficiency and complicate on-time departures and arrivals.

Absences among controllers and security checkpoint workers rose during the last government shutdown in 2019, which lasted 35 days, Reuters noted, as employees began skipping work after not getting paid.

Airlines, aviation unions and other members of the aviation industry say a government shutdown would force the Federal Aviation Administration to pause the hiring and training of air traffic controllers amid a national shortage.

The United States is facing a shortage of 3,000 air traffic controllers and is having difficulty keeping trainees because of the grueling work, according to the news service.

A meeting at the White House on Monday between President Donald Trump and congressional leaders on a short-term government funding bill failed to make much progress, leaving lawmakers pessimistic about reaching a deal to avoid a shutdown by Tuesday’s deadline.