Ryanair CEO Michael O'Leary said Wednesday that the risk of a no-deal Brexit grounding flights across Europe is being "underestimated."

Aviation is one sector that could be the most severely impacted after Britain leaves the European Union next spring. That's because there is no default fallback option for the industry if there is no deal on future ties.

The U.K. has said it wishes to remain part of the European Aviation Safety Agency (EASA) after leaving the bloc on March 29, 2019 but it is not yet clear whether this will be possible.

"We remain concerned at the increasing risk of a hard (no-deal) Brexit in March 2019," Ryanair's O'Leary said in a statement Wednesday.

"While we hope that a 21-month transition agreement from March 2019 will be agreed, recent events in the U.K. have added uncertainty, and we believe that the risk of a hard Brexit (which could lead to flights being grounded for a period of days or weeks) is being underestimated," he added.

In the absence of an agreement for the aviation industry, a no-deal Brexit scenario could result in flights being grounded. It would also mean Britain would need to takeover safety and regulatory processes currently being managed by the EASA.