Ryanair is seeking a licence to operate domestic UK air routes which it says it may need in the event of a hard Brexit.
The airline said it had applied to UK regulator, the Civil Aviation Authority, (CAA) for a UK Air Operator's Certificate.
Ryanair chief executive, Michael O'Leary, has repeatedly warned about the effect of Brexit on aviation.
He previously warned that flights between the UK and the EU could be grounded for months after Brexit.
"Ryanair today (2 January) confirmed that a subsidiary company Ryanair UK filed an application on 21 December last for an Air Operator's Certificate (AOC) with the Civil Aviation Authority in the UK.
"This may be required for Ryanair's three UK domestic routes in the event of a hard Brexit in March 2019."
Ryanair has a relatively small number of UK domestic routes.
Some of those routes have been suspended over the winter as part of a wider grounding after Ryanair admitting "messing up" pilot holiday rosters in September.
The airline was the censured by the CAA, which threatened Ryanair with legal action for "persistently misleading" passengers about rights.
The action came after Ryanair cancelled a further 18,000 flights in September.