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.
The British government said Monday that a no-deal Brexit could cause disruption to air travel between the U.K. and EU countries.
The scenario was set out in a batch of documents where U.K. and EU-licensed airlines would lose the right to fly to and from each destination without prior permission. The existing agreement is known as "open skies."
British officials said it would expect each side to allow each other to continue the access, but noted that "if such permissions are not granted, there could be disruption to some flights."
Britain is due to leave the EU on March 29, 2019. Both sides have yet to agree many aspects of their future relationship, despite fixing an October deadline to set terms.
Aviation bosses have been split over the lack of clarity in negotiations. In September, Michael O'Leary, CEO of Irish airline Ryanair, said that a no-deal Brexit was now more likely and in such a scenario, "flights will be grounded."
IAG, which owns British Airways, Iberia, and Aer Lingus, has been more positive in its assessment. Its chief executive, Willie Walsh, said in March this year that he firmly believed the issue of flying rights would be resolved.