LONDON — Ryanair on Friday proposed third-party mediation as its Irish pilots went on strike for a fourth day, disrupting flights at Europe's biggest no-frills airline.

The Irish carrier scrapped 24 flights out of 300 to and from Ireland on Friday, affecting 3,500 passengers, all of whom have been rescheduled or refunded, the company said.

A fifth day of wider industrial action is planned next week and two meetings with the Irish public service trade union Forsa have failed to resolve the differences completely.

"Ryanair now feels the only way to introduce common sense is by way of third party mediation," its chief people officer Eddie Wilson said in a statement.

The airline proposed Kieran Mulvey, the former head of Ireland's Workplace Relations Commission, as the mediator.

The Irish pilots, who took three days of industrial action last month, are demanding better working conditions.

However, the dispute has escalated since Ryanair announced plans last week to move planes from Dublin to Poland, which could cost 300 jobs, including 100 pilots.

On Thursday, pilots warned there would be a fifth day of strikes on Aug. 10, when the airline's Belgian and Swedish pilots will also be on strike. Pilots in Germany and the Netherlands are considering joining in.

Unrest surfaced at Ryanair following a planning mix-up in September 2017 which led to 20,000 flights being canceled. The crisis led to a U-turn at Ryanair, which started negotiations with trade unions in several countries.

Ryanair chief executive Michael O'Leary waived his hefty yearly bonus, according to the airline's annual report out Monday.

Ryanair condemned the planned fifth day of strikes. "This fifth strike notice by Forsa is irresponsible, unwarranted and is disrupting customers and (a way of damaging) Ryanair's business," Ryanair's chief marketing officer Kenny Jacobs said in a statement.

Forsa should explain why, "when Ryanair has agreed nine of their 11 requirements, they call strikes rather than negotiate", the statement said.

Jacobs said the strike was called by a quarter of Irish pilots.

"A handful of Aer Lingus pilots are working behind the scenes with pilot unions in Sweden, Holland, Belgium, and Germany to further disrupt Ryanair's business," he said.

Meanwhile, a Spanish union said Friday it was suing Ryanair over "irregularities" in the contracts of its Spain-based pilots, adding to the low-frill airline's woes as it is hit by strikes.

In a statement, the SEPLA pilots' union said it filed the lawsuit at Spain's top-level National Court after a year of negotiations with Ryanair to employ its members under Spanish rather than Irish legislation failed to bear fruit.

"Ryanair's refusal to recognize SEPLA and to apply the Spanish labor legislation to its pilots operating in Spain forces the union to use the judicial route to achieve this objective," it said.

Ryanair has said it will shift jobs and planes to more profitable areas, and threatened to move more if the strikes continue. Founded 33 years ago in Dublin, the airline boasts lower costs per passenger than its competitors. But employees have long slammed their working conditions.

Chief among complaints of workers based in countries other than Ireland is the fact that Ryanair employs them under Irish legislation, arguing most of its employees work on board Irish planes and are therefore covered by Irish law.

But staff counter that this creates huge insecurity for them, blocking their access to state benefits in their country and stopping them from applying for mortgages.

Last month, a Dutch appeals court ruled that Ryanair employees who are based in the Netherlands were covered by Dutch law. The airline had also until recently refused to recognize unions, but is gradually doing so as pressure increases.

"The recognition of trade unions and the regularization of the situation of pilots are not incompatible with efficient management in a low-cost company," SEPLA argued.

"We have examples such as Norwegian or easyJet, companies that hire their pilots based in Spain under the legislation of the country."

Ryanair refused to say anything on the lawsuit, saying it did not "comment on rumor or speculation." — AFP