Ryanair faced its worst one-day strike on Friday after a walk-out by pilots in five European countries disrupted the plans of an estimated 55,000 travelers with the budget airline at the height of the summer holiday season.

Ryanair, which averted widespread strikes before last Christmas by agreeing to recognise unions for the first time in its 30-year history, has been unable to quell rising protests over slow progress in negotiating collective labour agreements.

In response to unions serving strike notices, Ryanair had announced the cancellations in recent days of 250 flights in and out of Germany, 104 to and from Belgium and another 42 in Sweden and its home market of Ireland, where around a quarter of its pilots were staging their fifth 24-hour walkout.

The airline expected the travel plans of 42,000 travelers to be hit by the action in Germany alone, with the majority of passengers switched to another Ryanair flight and the remainder either refunded or rerouted.

"What I find unjustified is that the pilots draw the short straw, because people want to fly cheaply," said Daniel Flamman, one of several passengers Reuters spoke to at Frankfurt airport who said they sympathized with the pilots. "It's annoying that it's happening in the summer holidays, but it's the only means they have."

Ingolf Schumacher, pay negotiator at Germany's Vereinigung Cockpit (VC) union, said pilots had to be prepared for "a very long battle" and that it could take months to push through change at Europe's largest low-cost carrier.