The outspoken airline chief met Transport Secretary Chris Grayling today to discuss the impact of Brexit.
Mr O’Leary warned European airlines will be opposed to any favourable deal to secure air routes for Britain.
He said the German and French airlines will be looking to “screw the competition”, and summer flights in 2019 could be cancelled.
The single market for aviation, created in the 1990s, means there are no commercial restrictions for airlines flying within the EU.
Mr O’Leary said European airlines want a bilateral agreement enforced by the European Court of Justice – which will be rejected by those committed to a “hard Brexit”.
He said airlines will “scream blue murder” if an agreement is not reached, and said Britons will be left with “the option of driving to Scotland or getting the ferry to Ireland” for their holidays.
“There’s huge upside for German and French airlines in disrupting British Airways flights”
Ryanair boss Michael O’Leary
He joked that this will appeal to “all those many millions who like to go to Spain, Portugal and Greece for the cheap sunny holidays”.
Mr O’Leary said: “We think there’s nobody on the other side that will be that willing to facilitate giving the British a great deal by September 2018,” he said.
“There’s huge upside for German and French airlines in disrupting British Airways flights between the UK and Europe, disrupting easyJet’s flights… and causing us some grief that means we have to move planes out of the UK into continental Europe.
“If I was them I’d be doing exactly the same thing.
“If you can screw up your competition at reasonably little cost to yourself – they have very little capacity in the UK – then Godspeed.”