Europe's biggest budget airline is also expected to extend union recognition to cabin crew.
Ryanair has announced a 12 per cent rise in profits for the three months from October to December 2017, but has warned about "continuing uncertainty" over Brexit.
The results on Monday came one week after Ryanair signed an agreement to recognise the British Airline Pilots Association, reversing its historic hostility towards trade unions. It said post-tax earnings increased to €106 million ($132 million) year on year in the group's third quarter to December. "Our traffic will grow by 6% in FY19 to 138 million but very early indications are that summer 2018 fares will remain under pressure".
He said the decision to recognise unions "after 30 years of successfully dealing directly with our people" could add complexity to the business, and "may cause short-term disruptions and negative PR".
It said that the outlook for the remainder of 2018 was "cautious" and that although it maintained its full-year (profit) guidance in the range of €1.40 billion to €1.45 billion, this depended "heavily" on the "absence of union disruptions, unforeseen security events and close-in Easter bookings".
Ryanair has met pilot unions in Ireland, UK, Spain, Germany, Italy, Portugal, Belgium and France, with other countries to follow, and union recognition will be extended to cabin crew later in the year, Mr O'Leary said.More news: U.S. investigating possible sarin gas attacks in Syria: Defense Secretary Jim Mattis
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Ryanair has reported a 12% rise in quarterly profits in a "very challenging" period - but said it was less optimistic than rivals about the prospect of putting up prices for customers this summer. "In certain jurisdictions, unions representing competitor airlines will wish to test our commitment to our low cost, high pay/high productivity model to disrupt our operations".
The lack of clarity on Brexit continues to hang over the United Kingdom operations, said O'Leary, a prominent critic of the decision to leave.
Looking ahead to 2019, Ryanair said it has "practically zero visibility" on fares and does "not share the optimism of competitors and market commentators" for summer 2018 on price increases.
He added: "The lack of clarity on Brexit continues to overhang fares and pricing on routes to/from the UK".
It repeated its warning that there was "a worrying risk of serious disruption" to flights between the United Kingdom and the European Union from April 2019 unless there was some sort of agreement in advance of September 2018.