One of the world’s longest flights faces the potential chop unless Qantas Airways Ltd. and American Airlines Group Inc. get their way with U.S. regulators.

The airlines have asked the U.S. Department of Transportation a second time to approve a joint venture with antitrust immunity in their bid to deepen ties on flights across the Pacific.

Rejection could mean the end of Qantas’s marathon 17-hour Sydney-Dallas service, which requires a codeshare arrangement with American to be financially viable, the Australian carrier said in a statement Tuesday. The airlines argue that a knockback by the regulator would force them to reduce codesharing and focus only on what’s profitable for their own network.

Qantas has made super-long flights a cornerstone of its international strategy. It’s scheduled to start 17-hour flights between Perth and London next month. It has also challenged Airbus SE and Boeing Co. to build a jet by 2022 that can fly even further—from Sydney to London without a break.