Irish-based Ryanair pilots are to strike for a fourth time on August 3rd.
The Irish Airline Pilots' Association (IALPA) has warned further strikes may follow
Their union, Fórsa, said more action would follow later this month unless the airline's management "changed tack" and "negotiated in good faith" on the issue of base transfers and related matters.
In a statement, the union said it believed the airline's decision to issue protective notice to 300 of its staff on Wednesday was "an attempt to put pressure on its employees."
Ryanair earlier said it plans to reduce its Dublin fleet this winter which could see 300 pilots and cabin crew face redundancy.
It comes amid an ongoing row between the airline and the union representing Irish based pilots.
A view of a Ryanair plane at Lisbon Portela Airport | Image: NurPhoto/SIPA USA/PA Images
In a statement, Ryanair said the winter move is partly as a result of the strike.
It said these strikes have had "a negative effect on (close-in) high fare bookings and forward air fares as consumer confidence in the reliability of our Irish flight schedules has been disturbed."
Ryanair has issued letters of 90 days protective notice to over 100 pilots and over 200 cabin crew employees.
It said their services "may not be required" from October 28th onwards.
It comes as Ryanair has been forced to cancel 600 flights across Europe today and due to strike action by cabin crew in Spain, Portugal and Belgium.
The stoppage will affect tens of thousands of holidaymakers.