A vote for Brexit would be a hammer blow to Northern Ireland's airports, it has been claimed.
It would mean that EU flights would no longer be covered by a law that entitles passengers to compensation of up to £450 if there is an unacceptable delay.
And in Northern Ireland's case, this could potentially affect more than 80,000 flights annually - and could drive even more northern passengers to Dublin which, as an EU hub, would be unaffected.
Kevin Clarke, head of the flight delay team with Bott and Co solicitors, said: "In the event Belfast became a non-EU hub, it would be absolutely plausible for Ryanair or any other EU airline to set up flights from nearby Dublin Airport to the same destinations - but at a much, much lower cost.
"It would be difficult to see how Belfast could compete with that. This is a scenario unique to Northern Ireland because of its proximity to the border."
Flights which would no longer be covered include big names such as easyJet, Jet2, Thomas Cook, Thomson and Monarch, because they would no longer be EU carriers and passengers would no longer be departing from EU countries.
And transatlantic flights would not be covered in any direction.
Economist John Simpson said the future of Northern Ireland's airports - especially Belfast International - may be in jeopardy because Dublin will become a more attractive prospect.
"Being on a small island, any advantages in terms of doing your business from either the Belfast end or the Dublin end will be likely to be magnified," he said.
"In this case, if there are attractions in terms of better exchange rates or the type of legislation Dublin might sit with a major advantage, which would be to a disadvantage, particularly to Belfast International Airport."
Both Belfast International and George Best Belfast City Airports declined to comment.
The Civil Aviation Authority (CAA) confirmed there were 72,915 scheduled flights into and out of Northern Ireland in 2015, as well as 8,441 charters, totalling 81,356.
Flights from the EU, or on an EU airline to the EU, are currently covered by a European law that entitles passengers to compensation of up to £450 (or €600) if there is a delay of more than three hours.
That currently covers 94% of all flights into or out of the UK.
But if the UK votes to leave the EU this month that could all change, because people would no longer be flying from an EU airport and a lot of carriers would no longer be EU carriers.
Mr Clarke said it would affect around a million flights in and out of the UK every year.
"That's £400m worth of compensation that would be wiped out - and protection for over 100 million passengers would no longer be there," he added.
In the event of Brexit, flights on easyJet, Jet2, Thomas Cook, Thomson and Monarch would not longer be covered.
"If you were coming back into the UK on those airlines from an EU country, the regulation would cover you," Mr Clarke said.
"However, if you wanted to enforce your rights because the airline wasn't paying you compensation, you wouldn't be able to bring a claim under the UK legal system because it wouldn't recognise the regulation."
Flights operated by Ryanair - a huge proponent of staying in the EU - would be covered because its headquarters is in Ireland, which is an EU country.
We are on the brink of abruptly changing the core policy of how the UK has been governed for the past four decades. Here in Northern Ireland, while the polls show a clear majority mindful of the real benefits for Northern Ireland of being inside, not outside, the EU, and in favour of staying, the two top people in authority - the First Minister and the Secretary of State - are actively campaigning to take the UK and Northern Ireland out.