Ryanair has ordered some flight attendants in Spain to repay salary increases of up to €3,000 (£2,525) following a legal dispute with their union.

A pay rise had been agreed with the Spanish union, CCOO, and it applied to all flight attendants regardless of what union they belonged to. But another Spanish union – Union Sindical Obrera – won a court case in March to nullify the deal.

The Irish airline has now written to Union Sindical Obrera to say it will be informing its members how much they owe.

Union Sindical Obrera has hit back at Ryanair’s decision to claw back five month’s worth of pay rises, but the firm said: “USO are complaining about pay cuts that result from their court case.”

It added: “Ryanair is complying with the court case that Union Sindical Obrera took to cut pay while it is under appeal.”

The union told the BBC: “We are working on finding the most adequate legal solution for the matter.”

Ryanair has since agreed a pay deal for flight attendants who are members of the CCOO union.

They will not be required to pay back the increase they received between October and March, despite that original agreement being declared null and void by the court.

According to Union Sindical Obrera, the rises that Ryanair is demanding union members pay back range between €1,500 and €3,000.

Union Sindical Obrera also said that members and non-members have had their salary reverted to what it was before the agreement was implemented.

A spokesperson for Ryanair declined to answer questions on how many flight attendants it is pursuing or what will happen if they do not pay back the money.

But the Union Sindical Obrera said the airline has told the affected flight attendants “that the money will be simply deducted from the payslip until the debt is paid”.

It added that “some of the non-aligned workers are not forced to pay the amount back” but “all Union Sindical Obrera delegates have been requested to return the money”.

In a letter, Ryanair describes the union’s decision to maintain the terms agreed under the original deal with the CCOO, which the Union Sindical Obrera fought to nullify, as “farcical”.

However, the union maintains that those negotiating the deal were not authorised to do so and “legally couldn’t sign that kind of agreement which is why [Union Sindical Obrera] left the negotiations, after advising all other parties that the negotiations were irregular”.

In a document seen by the BBC, Ryanair informs a flight attendant, who is a member of Union Sindical Obrera, that because they are not part of the deal agreed with the CCOO this has “created an overpayment situation”.

The airline tells the worker that based on its calculations they were “overpaid” nearly €3,000.

It offers to let the flight attendant repay the money over a year by taking a sum out of their monthly salary.