Sky News has learnt that Ryanair tabled an application to the Civil Aviation Authority (CAA) shortly before Christmas for a UK Air Operator's Certificate (AOC).
Securing such a licence from British authorities may be required for the Irish carrier to continue operating its domestic UK business in the event of no deal being struck between ministers and their EU counterparts.
Although Ryanair's domestic UK traffic involves just three routes and accounts for less than 2% of total revenues, the AOC application underlines the ongoing contingency planning taking place at both British and EU-based airlines.
Ryanair's move follows a similar one by Wizz Air, the Central and Eastern European-focused airline, in October, to secure an AOC and operating licence from the CAA.
Some UK-headquartered airlines have been forced to take the opposite measure by applying for AOCs elsewhere to enable them to continue flights between EU countries after 2019.