"After a year of negotiations, the failure of Ryanair to recognise Sepla and apply Spanish labour legislation for the pilots who operate in Spain forces the union to take a legal path to attain this objective", the union said in a statement.
Trade union Forsa, which represents the pilots, hit out at the airline company following the warning.
The 24-hour stoppage will start next Friday August 10 at 1am.
Ryanair has condemned Fórsa for calling a fifth day of strike action on Friday 10 August.
"For over a month, the union has said that industrial action is likely to continue until there is substantial movement on the pilots' reasonable demands for an agreement on a fair and transparent approach to base transfers and related matters", Fórsa said.
Cabin crew in Italy, Spain, Portugal and Belgium staged a two-day strike that forced Ryanair to cancel the flights of more than 50,000 customers, the airline's biggest-ever strike.More news: Switzerland: Two plane crashes claim several lives
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Twenty-five per cent of Ryanair's crew are striking today, marking its fourth strike so far.
"Even when Ryanair invites FORSA to more meetings, FORSA just calls more strikes".
German Ryanair pilots have also voted to strike but are yet to announce the date.
Ryanair was unable to avert its first ever pilots strike on December 22 when pilots in Germany held a four-hour walkout with little impact on flights.
Pilots in Germany and the Netherlands are considering joining in.
"Forsa again rejected Ryanair's repeated offers to meet to resolve this dispute on Tuesday next", it said.
"This fifth strike notice by Forsa is irresponsible, unwarranted and is disrupting customers and (a way of damaging) Ryanair's business", Ryanair's chief marketing officer Kenny Jacobs said in a statement.