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A strike by Ryanair pilots is set to go ahead on Friday resulting in the cancellation of 24 flights between the Republic of Ireland and the UK.
It is the second of three 24-hour strikes involving Ireland-based pilots directly employed by Ryanair.
The pilots' union stated that talks on Wednesday between the two sides did not lead to any "material change".
Ryanair said if a strike planned for next week was not called off it would cancel 16 flights to and from Ireland.
The Irish Airline Pilots' Associationwhich is part of the Fórsa trade unionis 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 posted on Twitter a copy of a letter from its chief people officer Eddie Wilson to the national secretary of Fórsa.
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 stated 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."
He added that if Ryanair did not hear back from the union by 19:00 BST on Thursday calling off "this unnecessary strike", which he said was supported by by "just 25% of our Irish pilots", then the airline would cancel 16 flights out of more than 290 to and from Ireland next Tuesday.
"We can readily re-accommodate this small number of customers on other flights," added Mr Wilson.
Ninety-nine percent of the airline's directly-employed Republic of Ireland-based pilots voted in favour of action, but Ryanair stated 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 posted on Twitter that more than 85% of them had been "re-accommodated" on alternative flights or had applied for full refunds.
Update: Over 85% of our 50,000 customers whose flights on July 25th and 26th were cancelled due to a strike by some cabin crew in Belgium, Portugal and Spain, have been re-accommodated on alternative flights, or applied for full refunds.
— Ryanair (@Ryanair) July 19, 2018