O’Leary, the Irish low-cost airline’s chief executive, told reporters in London this morning he believed negotiations between Westminster and Brussels had "gone nowhere" since they began last month.

“My fear is that [customers’ summer 2019] holidays will be ruined if there is no agreement in place,” he warned.

He was speaking following a meeting with transport secretary Chris Grayling – the pair’s first since the general election - to “emphasise the importance” of securing a bilateral open skies aviation agreement before the UK leaves the EU in March 2019.

O’Leary described how the exit date felt like “a guillotine” over the heads of the aviation sector due to the perceived lack of movement on securing an aviation deal and told journalists that if no such deal was struck by next autumn, Ryanair would cease flying UK-EU routes and instead would operate between EU nations and issue refunds for British travellers booked to travel.

He said: "There is no fall back. The UK government doesn’t have a plan B. We’re going to need to know by September or October 2018 otherwise we’re going to start cancelling flights or taking flights off sale."

O’Leary refused to divulge the content or outcome of his private meeting with Grayling but added that he believed Grayling was “quite clear” about the importance of an agreement and was on “red lines” on the issue.

While discussing cooperation between the UK, the EU and European carriers, O’Leary claimed he had seen a leaked “briefing document” sent around a number of major European airlines including Lufthansa and Air France-KLM, the content of which made him believe the airlines were “hell-bent” on “screwing around” with British carriers.

“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,” he said.