The decision is a blow for Carles Puigdemont, who lives in self-imposed exile in Belgium, because it could pave the way for him to eventually be extradited to Spain, as Spanish authorities have requested for the past six years.

He fled Spain in 2017 to avoid prosecution following a short-lived declaration of independence that prompted Madrid to take direct control of the wealthy region.

In its ruling, the Luxembourg-based court rejected the appeal filed by Puigdemont and backed the European Parliament's decision to waive his immunity and that of two other self-exiled Catalan separatist MEPs, Toni Comin and Clara Ponsati.

"The Parliament cannot adopt decisions to defend immunity which produce binding legal effects with regard to the Spanish judicial authorities," the court said.

Puigdemont said on Twitter the three MEPs would appeal to the Court of Justice of the European Union.

"We will defend our fundamental rights until the end," he tweeted.

In January, Spain's Supreme Court dropped sedition charges against Puigdemont after a reform of the country's penal code abolished the crime.

However, he still faces charges of disobedience and embezzlement, which carry jail terms of up to eight years. Sedition carried a maximum jail term of 15 years.

Supreme Court judge Pablo Llarena said he would submit a new extradition request to Belgian authorities for Puigdemont to face trial on the lesser charges, depending on the EU courts' rulings.

Spain's previous attempts to have Puigdemont extradited during his stays in Germany, Belgium and Italy have failed.

Spain amended its penal code last year to remove the old sedition law under which some separatist politicians were sentenced to up to 13 years in prison after Catalonia's failed 2017 bid for independence. Prime Minister Pedro Sanchez later pardoned those convicted over the events.