The International Air Transport Association (IATA) said that 95 percent of flights between the UK and Spain could collapse in a no-deal scenario.
IATA, which represents 290 airlines globally, said the routed between Spain and the UK were particularly at risk because of the large number of tourist traffic between the them.
Alexandre de Juniac, IATA’s director general and chief executive, warned that a no-deal Brexit could lead to “thousands, millions of passengers” potentially grounded at airports after the UK leaves the EU on 29 March 2019.
“If nothing is done it will be a nightmare in the European and UK airports,” he said, reported The Guardian newspaper.
There are currently 5,052 flights operating between UK and Spanish airports each week.
During 2017, 2017 just over 82 million tourists visited Spain, with Britons forming the largest single contingent of tourists.
Air travel between nations is regulated by a hugely complex framework covering safety, security and immigration controls, meaning a no-deal Brexit could leave the UK out of all EU-negotiated agreements.
The UK’s Civil Aviation Authority have said they have negotiated a “bare bones” agreement with the EU to put in place a minimal level of service after March 29, if Britain crashes out without a deal.