Ryanair boss Michael O'Leary warned on Tuesday of a two to three-year economic downturn if Britain votes to leave the European Union.

Ryanair boss Michael O'Leary warned on Tuesday of a two to three-year economic downturn if Britain votes to leave the European Union and said that Brussels has been given a "wake-up call" regardless of the result of Thursday's UK referendum.

Speaking at an aviation conference in Paris, the head of the Irish budget carrier -- Europe's largest airline by passenger numbers -- said that economic and political disruption could weaken the rest of the 28-nation bloc if UK voters back a so-called Brexit.

"If Brits vote to exit, I think the European Union as a project is doomed," O'Leary said. "I think they will inevitably be followed by others. I think you would have a two or three-year economic downturn with huge uncertainty that would be very bad, not just for the UK economy but for the European economy, which is already struggling."

The Irish entrepreneur is one of the most vocal business leaders urging voters to back continued European Union membership in Thursday's referendum. Both sides in the closely fought contest claim significant business support.

"In my business it depends very much on what the exit looks like," O'Leary told the Paris Air Forum, hosted by La Tribune.

But he suggested that a vote to leave would penalise a generation that grew up on cheap connectivity to the rest of Europe and put at risk a system of unrestricted routes on which Ryanair and other budget airlines have built rapid growth.

Analysts say it is still unclear whether Britain, Ryanair's biggest single market, would continue to have unfettered access to EU airports under complex aviation rules.

O'Leary was speaking before travelling to London to join the climax of the campaign, alternating between defending membership of a reformed EU and criticising its bureaucracy.

"Even if Britain votes to remain, Europe and certainly Brussels has had a wake-up call," he said, adding that a clear message has already been sent to the EU's bureaucracy.

"If you guys in Brussels don't ... start improving the single market and improving the lives of Europe's citizens, this thing is going to fall apart."

Ryanair has often clashed with the European Commission over passenger compensation, taxes and other issues.

It is now lobbying for a clampdown by European authorities on repeated air traffic controller strikes.

Willie Walsh, head of British Airways owner IAG, has been more cautious about the impact of a Brexit, saying that it would "not have a material impact" on his business. – Reuters