Ryanair will put a 'Brexit clause' into all ticket purchase terms and coditions for summer 2019, in case a UK-EU deal on aviation is not agreed before the UK leaves the bloc.
The airline will warn all passengers that in the case of no deal, their tickets will not be valid. Customers will be refunded if this is the case, the airline said.
The EU-wide 'Open Skies' agreement currently covers flying rights to, from and within the EU, as well as between the US and UK. The deal allows EU airlines to fly to and from anywhere in the bloc. There is no backup option for this arrangement, which has been put in jeopardy by Brexit.
Airlines have said they need a bilateral replacement deal by October 2018 to be able to schedule flights beyond March 2019, when the UK will leave the EU.
"We'll announce our  summer schedule soon enough... and there'll be a term and condition that this is subject to the regulatory environment allowing this flight to take place," said Kenny Jacobs, Ryanair's chief marketing officer.
"We don't see a regulatory solution yet...If in the meantime between now and September, there's a regulatory solution found, then those tickets will be sold as normal."
Although ministers have said they expect to be included in the Open Skies agreement and the European Aviation Safety Agency, airlines have started drawing up contingency plans.
In October 2017, Thomas Cook said it would not be liable to pay compensation or reimburse expenses for delays caused by Brexit-related airspace closures, although it will refund tickets. Ryanair has also applied for a UK licence in a bid to soften the impact of a no-deal Brexit.
Jacobs said he did not expect Ryanair's Brexit clause to be needed. He explained: "There's clearly going to be an extension to Brexit," noting it was likely that the current rules would continue beyond March 2019.
"They are going to find a solution to Open Skies, but it will take a bit longer," he added.