The CEOs of America’s largest airlines have warned of “catastrophic disruption” to travel and shipping operations if telecommunications companies deploy 5G technology as planned on Wednesday without limiting the technology’s use near US airports.

Verizon and AT&T have already delayed the launch of their new C-Band 5G service twice due to warnings from airlines and aircraft manufacturers that the new system could interfere with altitude-measuring devices on planes.

“We are writing urgently to request that 5G be implemented throughout the country except within the approximate two miles of airport runways as defined by the FAA on January 19, 2022,” the CEOs wrote in a letter on Monday. The executives, in a letter to Transportation Secretary Pete Buttigieg and other U.S. government officials, emphasized the risk of “economic calamity” if Verizon and AT&T deploy the new technology before the necessary upgrades and changes to aviation equipment have been made.

“To put it bluntly,” they said, “the nation’s commerce will grind to a halt.”

The FAA announced on Sunday that it had approved some transponders to be safely operated within 5G deployment areas, clearing “as many as 48 of the 88 airports most directly affected by 5G C-Band interference.”

However, airlines are concerned that remaining restrictions at those airports, as well as a large amount of uncertified equipment, could spark a crisis, resulting in the cancellation of thousands of flights. “In addition to the chaos caused domestically,” the letter continues, the lack of certified planes “could potentially strand tens of thousands of Americans overseas.”

According to a news agency, carriers were considering canceling some international flights scheduled to arrive in the United States on Wednesday late on Monday.

United Airlines said in a statement, “The federal government’s current 5G rollout plan will have a devastating impact on aviation, negatively affecting an estimated 1.25 million United passengers, at least 15,000 flights, and much-needed goods and tons of cargo traveling through more than 40 of the country’s largest airports annually.”

In addition to the Transportation Secretary, the letter was sent to the FAA Administrator, the Chairman of the Federal Communications Commission, and the White House National Economic Council.

Airlines in the United States have also expressed concern about the potential costs. The executives urged officials to “take whatever action is necessary to ensure that 5G is deployed except when towers are too close to airport runways until the FAA can determine how that can be done safely without catastrophic disruption.”

The letter was signed by the CEOs of major airlines such as American, United, Delta, and Southwest, as well as the CEOs of shipping behemoths FedEx and UPS.

“Immediate intervention is required to avoid significant operational disruption to air passengers, shippers, the supply chain, and the delivery of critical medical supplies,” they wrote.

The FAA issued a brief statement Monday, ostensibly to assuage concerns about the impact of the 5G rollout on aviation, but it stopped short of announcing any concrete next steps in the process. “With safety as its primary mission, the FAA will continue to ensure the safety of the traveling public as wireless companies deploy 5G,” the agency stated.

“The FAA is continuing to collaborate with the aviation industry and wireless companies to reduce 5G-related flight delays and cancellations.”

Last February, Verizon and AT&T were awarded contracts worth tens of billions of dollars to operate 5G in the 3.7-3.98 GHz frequency bands, with service rollout set to begin on December 5.

Unless federal regulators intervene or the major telecommunications companies reach an agreement with the airlines, the major telecommunications companies are now scheduled to launch nationwide 5G service on January 19.