"There is a real prospect, and we need to deal with this, that there are going to be no flights between the UK and Europe for a period of weeks, months beyond March 2019," O'Leary said, according to the Telegraph.
O'Leary points to the Open Skies agreement, which currently allows European airlines to fly into all European countries, including Britain. But with Britain out of the E.U., the current agreement would become an "impossibility."
O'Leary said that if Britain adheres to the referendum voted on by Britons and breaks away from the E.U. as scheduled in 2019, European airlines will be restricted from flying into Britain.
But O'Leary, who was a staunch opponent of Brexit, said his airline won't accept any quick fixes.
"There is not going to be an interim agreement, there is not going to be a legal basis, we will be canceling flights, we will be canceling people's holidays for summer of 2019," he said.
O'Leary also quipped that Britons will quickly get bored with vacations close to home.
"It will be a couple of months before British people understand that they don't all want to go on holidays in Ireland or in Scotland, and I think the British government will be forced to come to its senses," he said.
Ryanair has been warning of canceling a slew of flights between Britain and E.U. Post-Brexit for several months.
In March, Ryanair's CFO, Neil Sorahan said the airline needed "clarity" on whether Britain would remain in the Open Skies agreement.
"It looks like they're not and if they don't then we need to see some movements made in relation to negotiating bilateral agreements," he said. "If these bilateral agreements are not negotiated by October 2018 then the chances of being rectified by March 2019 are very slim, which means there's a distinct possibility that there could be no flights in and out the U.K. for a period of time ... days, weeks, months, we don't know."
(Ray Downs - United Press International)