The Ryanair boss says "it's odds against a deal being done" before the end of 2018
Michael O'Leary has warned it is now 'more likely' there will be disruption to flights as a result of Brexit.
The Ryanair CEO was speaking after talks with the British government over a future 'open skies' deal between the EU and UK.
Speaking to reporters in London, Mr O'Leary warned: "It is now more likely there will be a disruption to flights. I think there will be disruption around September, October 2018.
"It's odds against a deal being done in advance of Christmas 2018 because it is in the Europeans' interest to not have a deal done, and all hell will be kicking off over here in the UK."
In comments quoted by Sky News, he also suggested Ryanair and other airlines "will have to start cancelling flights or taking flights off sale for the summer of 2019" if no deal is reached.
According to Reuters, meanwhile, he also claimed that some European airlines were against a positive deal for the UK, suggesting: "We believe the French and the German airlines in particular are actively negotiating against any favourable deal,"
Mr O'Leary was speaking after what he described as "a very good meeting" with the British Transport Secretary Chris Grayling.
The Ryanair boss noted: "We emphasised the need that there has to be an agreement reached by the UK by about September, October 2018."
Mr O'Leary has been one of the most prominent campaigners pushing for an early Brexit deal on aviation, and has warned on several occasions this year of potential disruption if a deal is not reached.
Although Britain is not scheduled to formally leave the EU until March 2019, O'Leary and others have called for early clarification as flight timetables are set months in advance.