The possibility of the UK leaving the European Union hangs heavy over the world, and Europe’s low-cost carriers could be severely affected by changes to border crossing regulations and other legislative changes.

“One of the things that UK citizens can understand that Europe has delivered for them has been the low fare travel revolution, cheap holidays abroad all over Europe, which is why so many young people are in favor of staying in,” said Ryanair CEO Michael O’Leary in an interview on CNBC. “If you take the leave side… we may not be free to fly anymore between the UK and Europe as an Irish airline.”

The UK has been an exceptionally strong market for both Ryanair and European low-cost carriers at large. Low-cost carriers will likely move away from the UK if it votes to leave the EU, focusing efforts on other popular European destinations.

Since it will take years to sort out individual trade deals between the UK and European partners, the continued expansion of London as a travel hub will likely end.

Easyjet, for instance, would likely set up a completely separate business in mainland Europe if UK leaves the union, according to Reuters.

“If UK votes to leave, it will be damaging for the UK economy and Europe economy for the next two or three years, there will be great uncertainty as the UK attempts to negotiate an exit out of a single market and replace that with a series of trade deals that won’t get done,” said O’Leary. “I want to keep growing the business here in Britain, but I can only do that if its a member of the European Union.”

Photo Credit: Ryanair CEO Michael O'Leary discusses the impact of the Brexit on Europe's low-cost carriers on CNBC. CNBC