Ryanair is to close its Spanish bases in Las Palmas, Tenerife South, Lanzarote and Girona from January, according to the local USO union.
It said the move would put the jobs of more than 500 pilots and cabin crew at risk.
The airline confirmed that bases will be cut or closed this winter due to the grounding of the Boeing 737 MAX jets.
In a statement, a spokesperson for Ryanair said, "As announced on July 16 due to the late delivery of up to 30 Boeing MAX aircraft this winter a number of Ryanair bases will be cut or closed this winter. These consultations are taking place with our people at affected bases currently. No routes will be affected as they will be served by flights from other bases from November when the winter schedule starts."
Ryanair confirmed the closures to the USO union in an email today and also announced the opening of a collective dismissal process, the union said in a statement.
It said 512 workers could be affected by the decision at the four airports, three of which are based in Spain's Canary Islands and the other on the mainland.
Ryanair currently operates around 2,400 daily flights from 87 bases.