A strike by almost 100 Ryanair pilots based in Ireland this Thursday "looks inevitable", according to trade union Forsa.
Representatives said there is little hope of a compromise at this point, as there was no contact between Ryanair management and unions over the weekend.
However, Ryanair still doesn't know which flights it will have to cancel during the 24-hour strike.
The airline operates dozens of flights in and out of Dublin alone on a typical summer's day, carrying thousands of passengers.
Ryanair, which has Michael O'Leary as its chief executive, has also scrambled to find replacements for the pilots who will be striking this week.
It has urged cockpit crew who aren't striking and who aren't rostered for work that day to volunteer for duty.
"In order to minimise disruptions to our Irish customers, we need to plan what services we can operate next Thursday," it told pilots in a circular.
"So we need to know in advance who will attend, or volunteer for duty next Thursday.
"We hope to maximise the number of flights to and from Ireland to help our customers and their families, many of whom will be flying on their annual summer holiday."
Pilots have been asked to respond to the plea by 9am today.
Ryanair pointed out that just 94 of its 350 pilots based in Ireland voted to strike, although only 95 of those were actually eligible for the ballot.
They are members of the Irish Air Line Pilots' Association (Ialpa) - a division of trade union Forsa - whereas the other pilots are employed as contractors.
Ryanair's chief people officer Eddie Wilson has branded the planned strike as "blackmail" in a letter to staff.
He claimed the planned action has "no democratic mandate whatsoever".
The pilots are striking over terms and conditions, including a demand for transparency in relation to how first officers are promoted to captain.
Ialpa has complained that Ryanair's current system of promoting first officers typically includes a mandatory base change, even if there are captain roles available in the first officers' existing bases.
Ryanair has insisted that it has addressed the issues raised in written proposals sent to Forsa.
The airline has also said that it remains ready to thrash out issues with the union at the airline's headquarters any day before Thursday.
However, Forsa and Ialpa have insisted that any such meeting be held at a neutral venue, and a Forsa spokesman yesterday confirmed that this had not changed.
He also confirmed that there had been no contact between the sides over the weekend and that the strike now looks inevitable.
Ryanair has urged Forsa to cancel the planned strikes as it also faces strike action by cabin crew in Italy, Spain, Portugal and Belgium over demands for concessions in relation to their conditions of employment.