Strike action which could last until 2023 and cause major disruption across Europe has begun.
Ryanair cabin crew members in Spain are walking out Monday to Thursday every week from now until January 7, marking five months of industrial action.
They are demanding better pay and working conditions from the budget Irish airline, a leader of the USO union has said.
The strike action, which is also backed by the SITCPLA union, follows several previous walkouts by workers at Ryanair in Spain.
Ten Spanish airports were affected during those strikes, including Madrid, Malaga, Barcelona, Alicante, Sevilla, Palma, Valencia, Girona, Santiago de Compostela and Ibiza, with the same airports likely to be targeted again.
While British airports won't be targeted specifically, flights from the UK arriving in Spain or travelling the other way risk being delayed or cancelled.
The 18 days of strikes before this latest period of action caused an estimated 310 cancellations and about 3,455 delays at 10 Ryanair bases in Spain.
"As the company has been unable to listen to the workers, we have been forced to call new strike days," said Lidia Aransanz, a leader for USO's Ryanair section.
Ryanair has played down the likely impact of the strike action and said it expected minimal disruption in Spain this winter.
“These two tiny unions who represent only a handful of our Spanish cabin crew have held a number of poorly supported “strikes” in June and July which have had little or no impact on Ryanair’s flights to or from Spain," it said in a statement.
"In July alone Ryanair operated over 3,000 daily flights and carried a record 16.8m passengers – many of them to/from Spain. Ryanair expects that these latest threatened strikes, which involve only a handful of our Spanish cabin crew, will have zero impact on our Spanish flights or schedules in August or September
"On a point of correction, while a tiny number of Ryanair flights in Spain were cancelled or delayed in July, this was mostly due to ATC strikes and flight delays.
"No flights were cancelled in July due to these unsuccessful and poorly supported strikes by these two minority unions (USO & SITCPLA) who represent only a small handful of Ryanair Spanish cabin crew.
"The vast majority of Ryanair’s Spanish cabin crew are represented by the CCOO union who have already reached a labour agreement with Ryanair which covers most of our Spanish cabin crew.”
Ms Arasanz said Ryanair cabin crew in Spain have been calling on the company to honour contract agreements since 2019.
“We used to be on Irish contracts, but in 2019 Ryanair signed an agreement to pass all workers under Spanish law,” she told Euronews.
“But they have only implemented the parts that they want.”
The union is asking the airline to align workers’ contracts with Spanish law, including giving employees 30 days of annual leave, 14 bank holidays per year and two extra payments most Spanish workers receive annually.
According to the union, 11 employees were dismissed and around 100 are facing disciplinary action for their involvement in the strikes.
This week's strikes are the latest in a summer of industrial action in the aviation industry.
Many airport and airline workers have found themselves working flat out as passenger numbers return to near pre-pandemic levels.
The easing of coronavirus restrictions led to a sharp rise in demand which airlines and airports - having slashed staff numbers during the lockdowns - have struggled to meet.
This has caused tensions to rise among overworked and underpaid employees including check-in staff, cabin crew and pilots over the last few months.
A strike of refuelers at Heathrow airport was diverted at the end of July when a last minute pay agreement was tabled.
SEPLA has said there will be three 72 hour strikes of easyJet pilots in Spain taking place from 12 to 14, 19 to 21 and 27 to 29 August.
In Portugal, aviation staff from operator ANA has planned strike action to impact Lisbon, Porto and Faro from August 19 to August 21.
Cabin crew from Spanish airline Vueling are threatening to walk out in a dispute over wages.
They could be joining pilots from Lufthansa, who voted in favour of industrial action last week.
Ryanair, Wizz Air and British Airways staff have all spoken to the Mirror about issues they've had since returning to work.
Many pilots and cabin crew members spoke of high levels of fatigue and stress in the ranks, with some struggling to make ends meet in their personal lives as the cost of living crisis continued to bite.
