The EU's system of digital COVID travel certificates comes into force on Thursday (July 1).
It's meant to enable travel for people who can show they are fully vaccinated, have recently recovered from the virus, or have had a negative test.
Passengers will be able to show a QR code to prove their status.
But in a letter to EU leaders, airlines and airports have warned that the system could cause chaos.
They say the system is too fragmented, with different practices in different countries.
The industry says vaccination certificates other forms must be processed before travellers arrive at airports, or there will be huge delays.
They want governments, not airports, to manage all the data and provide equipment for reading the codes.
Air travel organisation IATA says the time take to get through an airport during a journey has already doubled to three hours.
It warns that could rise to five or even eight hours without changes to current procedures.
European Commission guidance on the new scheme echoes some of the concerns.
Brussels has warned that 10 different combinations of checks were being planned by different countries.
Airports say any resulting chaos will just further undermine confidence in air travel.