Ryanair is making contingency plans for disruption to flights after pilot unions in three countries announced strikes for the coming days and two others threatened industrial action.
The strikes, which would be the first in Ryanair's 32-year history, are being organised by pilots demanding an overhaul of the company's system of contracts and collective bargaining.
The pilots says this gives management too much power.
Pilots and ground crew in Italy are due to take action first with a four-hour strike on Friday, to be followed by a 24-hour stoppage by pilots in Ireland and Portugal on December 20.
Pilots in Germany and Spain have said they are considering industrial action.
A company spokesman said: "Ryanair has been notified of threatened industrial action up to and including a 24-hour strike on Wednesday December 20 next by a minority of our pilots in Ireland, Portugal, Germany, Italy and Spain.
"This may lead to some flight disruptions."
The company will publish contingency plans to minimise disruptions on December 18, he said.
"We apologise sincerely to our customers. Rest assured that we will do everything we can to minimise disruption for our customers," the spokesman said.
Pilots at Europe's largest airline by passenger numbers have mobilised in the wake of the announcement of 20,000 flight cancellations by Ryanair, which it blamed on rostering issues.
Ryanair, which does not recognise trade unions, promised to "face down" the threatened strike.
Management has warned pilots in internal memos that pilots who take part in strike action may lose out on promotions and could cause the company to shift planes and jobs to lower-cost bases.
Portuguese union SPAC said in a statement that it "deeply regrets the failure by management to engage with the pilots' representatives in a meaningful manner".
It said its pilots had decided to strike because Ryanair was refusing to offer contracts under local rather than Irish law and to recognise the union's company council as its exclusive negotiating partner in Portugal.