Ryanair is to cancel 16 flights to and from the Republic of Ireland on Tuesday, affecting 2,500 passengers, because of a strike by pilots.
The action, by Ireland-based pilots. has already led to two 24-hour strikes, with 4,000 passengers being affected by flight cancellations on Friday.
The pilots' union said that talks on Wednesday between the two sides did not lead to any "material change".
The union is in dispute with Ryanair over pay and conditions.
The pilots' concerns are centred on Ryanair's proposals on seniority, as well as procedures for the allocation of base transfers, promotions and annual leave.
Late on Thursday, Ryanair tweeted a copy of a letter from its chief people officer Eddie Wilson to the national secretary of the Fórsa trade union, which includes Irish Airline Pilots' Association .
Ryanair calls on FORSA to call off 24 July strike by less than 25% of our Irish pilots (over seniority & base transfer proposals which they can't explain and which don't even affect them): pic.twitter.com/OVse5kmgMi— Ryanair (@Ryanair) July 19, 2018
In it Mr Wilson said that following Wednesday's meeting between the airline and the union "nothing has progressed".
"It is unacceptable that 24 hours later we have had no response from Fórsa, and find ourselves with a threatened third day of strike action next Tuesday."
On Friday, Ryanair tweeted that this strike action meant it would cancel 16 of more than 290 Irish flights on Tuesday, 24 July.
The carrier added that all 2,500 passengers affected had been notified by email or text message, and would be "readily re-accommodated (or refunded)" on other Ryanair flights over the next seven days.
Ninety-nine percent of the airline's directly-employed Republic of Ireland-based pilots voted in favour of action, but Ryanair said that they make up just 25% of its Irish flight crew.
Most pilots flying for Ryanair are self-employed, which is why the airline is able to continue most flights.
Separately, Ryanair cabin crew are also due to go on strike on 25 and 26 July.
The airline has cancelled up to 600 flights over what it calls the cabin crew's "unjustified" action.
Almost 50,000 customers flying from airports in Belgium, Portugal and Spain were affected, but earlier the airline tweeted that more than 85% of them had been "re-accommodated" on alternative flights or had applied for full refunds.