O’Leary insists passengers will be entitled to refunds in this instance, but not any flight compensation.
He said: “As long as we cancel flights as much as four weeks in advance of the date of departure, you clearly have to give people a full refund, but they don’t have any compensation entitlements.
“The real challenge for airlines is that period through to September, October, November, when the s*** will be hitting the fan all over the place.
“We don’t have certainty and we have to make sure our aircraft are on sale somewhere where we at least know they’ll be able to fly.”
If no agreement is struck by September 2018, Ryanair plans to cancel any booked flights beyond April 2019 when Brexit has been finalised.
Mr O’Leary said: “We will be taking flights that are scheduled from the UK to Europe and are on sale on our website for April 2019, we will be taking those flights off sale and issuing refunds.
“We’ll be putting those flights on sale between EU airports, Ireland to Spain; Germany to Italy; Greece to Scandinavia. We won’t be the only ones, everybody will be doing the same thing.
“That’s when you’ll have the Department for Transport or maybe even Boris Johnson explaining how exactly people are going to be going on holidays to Europe in summer 2019.”
The Government insists it is “committed to getting the best deal possible” for UK aviation in a post-Brexit world.
A spokesperson told Express.co.uk: ”Aviation is absolutely crucial to the UK’s economy and we are committed to getting the right deal for Britain.
“Our aviation industry is the largest in Europe, and both we and the EU benefit from the connectivity it provides.
“That’s why we are pursuing liberal access to European aviation markets – including all the benefits that brings for consumers.”
