Ryanair has warned flights between the UK and the European Union may be suspended even if an aviation deal is reached in Brexit negotiations.
The Dublin-based carrier claimed a delay in such an agreement being ratified by EU nations could result in flights being halted for several months.
The single market for aviation, created in the 1990s, means there are no commercial restrictions for airlines flying within the EU.
But a fresh deal must be secured if the UK's airline routes to the EU can continue after Brexit, which is scheduled to take place by March 2019.
Ryanair chief financial officer Neil Sorahan said: "We're looking at a scenario whereby today you can fly back and forth between the UK and Europe, tomorrow you can't.
"Whether that is for a number of days, a number of weeks, a number of months, nobody knows at this point in time.
"But when you're dealing with a block of 27 countries that have to ratify something that's been agreed at the centre, we all know that politics moves slowly."
An "even worse scenario" would be for no agreement to be reached at all, he told a press conference in central London, before adding that "seems highly unlikely".
Airlines want aviation to be prioritised during the Brexit negotiations as flights are scheduled more than one year in advance.
"The clock is ticking down all the time," Mr Sorahan warned.
"Nobody really knows where this is going.
"We need somebody to take control of the process and say 'this is what's going to happen, this is what we're going to do'.
"We need Prime Minister (Theresa) May to say 'we will absolutely agree to the free movement of people, we will absolutely agree to the rulings of the ECJ (European Court of Justice)'.
"We need that level of certainty so that ourselves and other airlines can make decisions and load flights for summer 2019 this time next year."
As an EU-based company, Ryanair expects it will need to apply for a UK operating licence after Brexit to continue domestic flights in the country, such as Belfast to London Gatwick and Glasgow to London Stansted.
But Mr Sorahan claimed the airline may prefer to cut the services completely.
He said: "It's a very small part of our business. It's less than 2% of our capacity.
"The decision may be made not to go for an AOC (air operator certificate) and just to abandon domestic UK flying.
"It's not something that we particularly want to do but it's something that could happen."
Luton-based carrier easyJet has said it will announce its application for an EU AOC shortly to allow it to secure flying rights for the 30% of its routes that do not touch the UK.
This will cost the airline an estimated £10 million.