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 there to be no agreement he told a London press conference, before adding that that seemed “highly unlikely”.
Airlines want aviation prioritised during Brexit negotiations as flights are scheduled more than one year in advance.
“The clock is ticking down all the time,” Mr Sorahan warned.
“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 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)’.