Mr O’Leary, chief executive of the Ireland-based airline, said the European Commission wanted to “yank the tail” of the UK – while foreign operators were determined to block a good Brexit deal.
Leaving the EU means Britain will automatically quit the EU Open Skies agreement which lets all flights operate between here and the continent.
That means the UK will have to strike a new deal with the other 27 member states so flights can continue after the Brexit deadline of March 29, 2019.
But Mr O’Leary said German and French airlines are “actively campaigning against any favourable deal for Britain”.
He also said: “The European Commission has worked out it can really yank the tail of the UK by screwing it around on flight rights.”
After having talks with Transport Secretary Chris Grayling, Mr O’Leary said UK-Europe flights would be grounded without a deal.
He said: “You will not have anybody flying between the UK and Europe – and that’s the issue the EU is going to hold over the UK Government.”
Mr O’Leary said he had seen documents put out by airlines including Lufthansa, Air France and KLM which call for any future flights to be under the jurisdiction of the European Court of Justice – from which the UK wishes to break free.
He added: “There’s no doubt in our minds that the German and French airlines in particular are opposed to the UK getting any favourable deal.
“They are in favour and are actively campaigning for a bilateral with the UK under which the UK must accept ECJ jurisdiction.”
He said this would make it “almost unacceptable” to those who do not wish to bend to European regulations.
Mr O’Leary added: “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.
“There’s a huge upside for German and French airlines in disrupting British Airways, disrupting easyJet and causing us grief.
If I was them I’d be doing exactly the same thing.”
Mr O’Leary said that without a deal Ryanair, as an airline from an EU state, would move its 70 UK-based planes to the continent – but BA and easyJet did not have that freedom.
Airlines and the Government dismissed his concerns, arguing that European operators, hoteliers and citizens want flights between Britain and the EU to continue.
A Government spokesman said: “Aviation is absolutely crucial and we are…