Ryanair passengers face a fourth pilots' strike over the August bank holiday weekend when up to 20 flights will be grounded.
More stoppages are being planned despite a threat by the airline - which is planning to axe 300 posts - that it could cut even more jobs after the latest strike announcement.
Around 100 Irish-based pilots will man the picket lines a week tomorrow in the increasingly bitter dispute over the pilots' terms and conditions.
The announcement came after the low fares carrier put more than 100 of its pilots and 200 cabin crew on protective notice of redundancy.
This means they could lose their jobs in October, when Ryanair shifts six aircraft from Dublin to Poland.
The bitter dispute over the pilots' terms and conditions deepened yesterday when the airline warned it could cut even more jobs after the latest strike announcement.
About 100,000 passengers have had their flights cancelled across Europe in the biggest strike in Ryanair's history.
There were reports that additional flights were grounded on top of 600 previously announced due to a 48-hour strike by cabin crew in Spain, Portugal, Italy and Belgium.
Ryanair did not respond to a request for a comment.
Insiders said some Irish-based pilots are set to apply for jobs at Norwegian Air due to the lay-off threat. They claimed some already have offers.
However, a message from Ryanair chief operations officer Peter Bellew sent to pilots later yesterday adopted a gentler approach.
It emphasised that not all those who got a notice would be required to transfer or be made redundant.
"Since at most 24 of our 30 based aircraft should remain in Dublin not everyone who receives a notice today will be required to transfer or be made redundant," it said.
"If you have an excellent performance record you will be less likely to be selected."
Union sources said they believed the job cuts were a threat rather than a promise.
"How is Ryanair going to make 100 pilots redundant when it doesn't have enough pilots as it is?" said a source.
Industry sources played down the significance of the threat.
They said the airline had always planned to expand its Polish charter business and there was nothing unusual about it cutting winter capacity.
Merrion Capital senior equity analyst Darren McKinley said Ryanair is sending out a statement to workers that "this is how we will handle strikes".
Ryanair is due to contact customers who will be affected by the August 3 strike today.