Ryanair has today cut its forecast for full-year profit for the second time in three months, this time blaming lower than expected winter fares.
The airline said it could not rule out a further downgrade if Brexit causes unexpected developments.
Ryanair shares fell after the profit warning, which also hit the stock prices of rivals such as EasyJet as the airline said short-haul overcapacity in Europe had led to fare cuts.
Europe's largest low-cost airline now expects profit after tax for its financial year to 31 March - excluding start-up losses at its Laudamotion unit - of between €1bn and €1.1bn.
This compares to a previous estimate of €1.1bn to €1.2bn.
The airline had originally forecast profits of €1.25bn to €1.35bn before October's profit warning took account of a series of strikes across Europe during the summer that hit traffic and bookings, but have since subsided.
The new range would represent a 24-31% fall from the record €1.45bn post-tax profit Ryanair booked in its most recent financial year.
In recent weeks, the company's shares have fallen to their lowest level in four years.
They were flat this afternoon after initially falling more than 5% in opening trade in Dublin.
EasyJet shares were 2% lower, while Lufthansa and Wizz Air were down 0.7% and 1.1% respectively.
Ryanair's profit warning came after another budget carrier, Norwegian Air, said on Wednesday that it would axe a number of routes and shut several bases as it seeks to cut costs.
In December, Norwegian said it was struggling to fill its aircraft as capacity growth far outpaced demand.
Ryanair said today that its fares in the second half of its financial year were set to fall by 7%, rather than the 2% previously flagged.
The lower fares have, however, been partially offset by stronger than expected annual traffic growth - now expected to grow by 9% to 142 million passengers - slightly better than expected unit costs and stronger ancillary sales.
Ryanair chief executive Michael O'Leary said a further downgrade of the profit outlook was possible given uncertainty about the terms of Britain's planned departure from the European Union at the end of March.
"While we have reasonable visibility over forward Q4 bookings, we cannot rule out further cuts to air fares and/or slightly lower full year guidance if there are unexpected Brexit or security developments which adversely impact yields between now and the end of March," Mr O'Leary said in a statement.
He said the fact that the airline was passing on lower air fares to customers would continue to be good for Ryanair's traffic growth and business over the medium to long term.