Ryanair has secured a UK operating licence to fly within the UK and UK to non-EU routes in the event of a no-deal Brexit.
The Civil Aviation Authority said it issued an air operator's certificate (AOC) to the low-cost carrier today.
The Dublin-based airline has expressed concern that flights could be grounded if the UK withdraws from the EU without an agreement.
Flights between EU nations and from the UK to many non-EU nations currently operate due to the UK's membership of the bloc.
UK Transport Secretary Chris Grayling has insisted the UK and the EU do not want flights to be grounded after Brexit "in any scenario".
Ryanair's company secretary Juliusz Komorek said: "We welcome the Civil Aviation Authority's decision to grant our UK based airline, Ryanair UK, with a UK AOC, allowing Ryanair to operate UK domestic routes and UK to non-EU routes in a post-Brexit environment.
"The risk of a no-deal Brexit in March is rising, and despite our robust post-Brexit structures, including our post-Brexit plan around European ownership, we continue to call for the UK and EU to agree a transition deal from 31 March 2019, so that any disruption to flights and British consumer summer holidays in 2019 can be avoided."
Ryanair has said it may restrict the voting rights of its non-EU shareholders in the event of a no-deal Brexit to help the airline comply with EU laws.