American pilots who fly worldwide for Deutsche Post DHL have voted nearly unanimously to authorize their union to call a strike if necessary.

Across all five of DHL’s North American network carriers – Atlas, Southern, Polar, ABX, and Kalitta – voting showed showed 99 percent of those pilots voted in favor of a strike. In total, 93 percent of the five-company pilot groups participated in the strike vote.

A strike could cripple DHL’s global operation as the five carriers account for approximately 70 percent of DHL’s total flying worldwide. A pilot strike could also have a significant impact on Amazon.com. In March, ATSG – which owns ABX – announced an agreement with the online retailer to operate an air cargo network serving Amazon customers in the United States. Meanwhile, Amazon.com just signed an agreement with Atlas Air to double its fleet for domestic packages.

Nearly 2,000 pilots at the five U.S. cargo carriers voted online through anonymous balloting in April and May. Pilots at Atlas, Southern, Polar, and ABX first called for the strike vote in March as concerns grew out of lingering contract negotiations with their individual carriers. Kalitta pilots authorized a strike in December 2015. The pilots are represented by the International Brotherhood of Teamsters and its airline affiliate, Teamsters Local 1224.

“This is truly an unprecedented situation where pilots across five different companies are standing up together to send a message that we refuse to be treated below industry standards,” said Captain Mike Griffith, an Atlas pilot. “By calling this strike vote, we are telling our respective companies – and DHL – that they need to listen to the pilots who keep them flying and help build their millions in profits.”

Atlas Air Worldwide Holdings (AAWW) owns Atlas Air, Inc., Polar Air Cargo, Inc. and Southern Air Holdings, Inc. Air Transport Services Group (ATSG) owns ABX.

The unions contend the carriers are trying to force pilots into sub-standard contracts that would suppress wages and lower quality of life.

The Kalitta pilots have been engaged in negotiations with the company for nearly six years. ATGS’s ABX pilots endured furloughs and wage and benefit concessions in 2009, when DHL cut its operations in Wilmington, Ohio. Since then, pilots have been working under the 2009 concessionary contract with ABX and have been negotiating for an amended contract for more than two years.

The Atlas pilots are currently in mediated negotiations, but pilots are increasingly concerned by AAWW’s and Atlas Air’s insistence that Atlas Air and Polar pilots stop ongoing contract negotiations and merge their contract with the Southern Air pilots’ existing contract.

The airlines have said that existing collective bargaining contracts call for a merger of agreements in the case of an acquisition by the company.