Ryanair would likely cut back its Glasgow, Edinburgh and Belfast routes in the event of a no-deal Brexit, according to chief executive Michael O'Leary.
Asked by analysts about the location of cuts planned in the coming months as a result of delays in the delivery of the Boeing 737 MAX, Mr O'Leary said the three routes would be obvious candidates.
"An obvious one (to cut) if there is a hard Brexit at the end of October, we have three UK domestic routes ... They would readily fall out straight away," he said.
"There is a longer-term question mark over those in a hard Brexit," Mr O'Leary said. "Frankly ... we are not growing in Edinburgh, Glasgow or Belfast."
The airline has reported a 21% drop in quarterly profit as overcapacity and a price war in Germany drove ticket prices lower, but the airline kept its profit target for the year unchanged.
Shares in Europe's largest low-cost carrier have almost halved in value in two years as the company deals with overcapacity, Britain's plans to leave the European Union and delays in the delivery of the Boeing 737 MAX.
In recent weeks, it has been also been hit the threat of strike action by pilots in Ireland and the UK and cabin crew in Portugal.
Chief Financial Officer Neil Sorohan declined to comment on the likelihood of disruption in the coming weeks, but described the timing of ballots for industrial action as "a little bit unusual" given concerns around the MAX and Brexit and said the airline was open to talks.
The Dublin-based airline reported a profit after tax of €243 million for the three months to 30 June, down 21% from €309m a year earlier.
That was ahead of a forecast by a poll of analysts published by Ryanair ahead of the results of €232m.
"The June quarter results were not quite as bad as feared," Liberum analyst Gerald Khoo said in a note, adding that ticket revenue was slightly light compared with forecasts but revenue from optional extras was better than anticipated.
Ryanair reiterated its profit forecast for the year to 31 March 2020 of between €750m and €950m, compared to a forecast of €832m in the analyst poll.
"Current FY consensus is already in the bottom half of management's guidance range, but there may be some slippage as higher estimates are reined in," Mr Khoo said.
Ryanair said its fares in the three months to the end of June declined 6% from a year earlier and said it expected a similar fall for the remainder of the summer.
Average fares for the year to end-March 2020 will be towards the lower end of its guidance range of -2% to +1%.
Ryanair's shares closed 1.4% lower in Dublin this evening.