The International Air Transport Association (IATA) has said there is still a "huge amount" of work to be done to avoid grounded flights after Britain leaves the European Union next year.
IATA is a trade association for the world's airlines, representing more than 80 percent of planes in the sky.
"The U.K. government's papers on the air transport implications of a 'no deal' departure from the EU clearly exposes the extreme seriousness of what is at stake and underscores the huge amount of work that would be required to maintain vital air links," Alexandre de Juniac, IATA Director General and CEO, said in a statement Tuesday.
"While we still hope for a comprehensive EU-U.K. deal, an assumption that 'it will be all right on the night' is far too risky to accept," he added.
De Juniac highlighted that it wasn't just permission for aircraft to take off and land that needed to be resolved, but also how to administer security clearances and pilot licenses.