Ryanair told staff last month it planned to cut its presence on the ground at airports due to delays in the delivery of the grounded Boeing 737 MAX, that has left it with 900 more pilots and crew than it needs.

The airline's chief people officer told Reuters last week that it would likely close its base in the Portuguese city of Faro and two on the Canary Islands but that a final decision had yet to be made on how many bases, jobs and routes would go.

Ryanair confirmed the closures to the USO union in an email on Friday 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, which currently operates around 2,400 daily flights from 87 bases, did not immediately respond to a request for comment.