Ryanair is seeking a licence from British regulators to safeguard against a hard Brexit amid little apparent progress on a new aviation deal between the UK and Europe.
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.
EasyJet is seeking a licence from Austrian regulators to establish a new airline in the country.
It is unclear whether Ryanair would be required to relocate a significant number of aircraft or employees to the UK in order to persuade the watchdog to grant the new AOC.
The airline currently operates a number of flights between London Stansted, and Belfast, Edinburgh and Glasgow airports.
In response to an enquiry from Sky News, a Ryanair spokeswoman said: "Ryanair today confirmed that a subsidiary company, Ryanair UK, filed an application on 21 December last for an AOC with the CAA in the UK.
"This may be required for Ryanair's three UK domestic routes in the event of a hard Brexit in March 2019."
The application follows months of warnings by Michael O'Leary, Ryanair's pugnacious chief executive, that British holidaymakers face widespread cancellation of flights to popular EU destinations without meaningful progress on a new aviation deal.
Mr O'Leary met Transport Secretary Chris Grayling last summer to discuss the state of negotiations about post-Brexit aviation rules.
"There is a real prospect, and we need to deal with this, that there are going to be no flights between the UK and Europe for a period of weeks, months beyond March 2019," the Ryanair chief told the European Parliament's Transport and Tourism Committee in July.
"There is not going to be an interim agreement... we will be cancelling people's holidays for summer of 2019."
While talks between the UK and EU are now progressing to a second phase focused on future trading arrangements, there remains uncertainty about the extent to which the aviation sector will be prioritised in discussions.
Other airlines have been more sanguine about the prospects of a deal being reached in time.
With ticket sales for flights commencing months in advance, though, industry executives believe a deal is essential well before the end of this year.
Analysis carried out by some of Britain's biggest airports last year suggested that between March 2018 and March 2019, there could be a 41% fall in passenger demand - equating to 8.1m bookings, or 16.2m journeys - between the UK and EU countries.
The CAA declined to comment, although a notice published on its website on Tuesday highlighted that it had received an application from Ryanair UK Limited for a Type A licence - the category required by large airlines.
A Government spokesperson said: "Aviation is crucial to the UK's economy and we are committed to getting the best deal possible for Britain.
"We will work closely with the international aviation community to ensure that this global industry continues to be a major success story for the UK economy."
Ryanair's application for a UK AOC requires the blessing of the CAA three months after the regulator instigated enforcement action against the airline for misleading passengers about their rights in relation to last autumn's deluge of flight cancellations.
An escalating staffing crisis at Ryanair before Christmas amid threatened strikes prompted the carrier to say that it would recognise pilots' and cabin crew unions for the first time.