Carles Puigdemont, the self-exiled former leader of Catalonia, wants to return to Spain for the first time since fleeing to Belgium.
But it might cost him his freedom.
Puigdemont, now an MEP, fled Spain in 2017 after organising Catalonia's illegal independence referendum.
He wants to go back to visit other ex-Catalan leaders jailed for their involvement in the vote.
His visit will be part of a mission with the European Parliament and is scheduled for the end of February.
Puigdemont and former Catalan cabinet member Toni Comin were elected to the European Parliament last May.
“What we most want to do, Toni Comín and I, is to visit the political prisoners," Puigdemont recently told a Catalan radio station. "I should be able to do it."
Puigdemont cited his immunity as an MEP as a reason to be allowed to visit Spain.
But on Thursday, the European Parliament, following a request from Spain, began a procedure over the possible lifting of this immunity.
The parliament said it could take several months for a decision.
The EU's top court recently ruled in his favour, allowing Comin and Puigdemont to take up their seats in the European Parliament despite being wanted in Spain.
But in practice, it isn't that simple.
A complex and unprecedented situation
In the eyes of Spanish justice, Puigdemont can be arrested.
Agustín Ruiz Robledo, a professor of constitutional law at the University of Granada, told Euronews that it as a "very complex" situation in which there are no precedents to rely on.
European law distinguishes between the immunity enjoyed by a MEP in his state and the one enjoyed in other states, he said.
"While they could not be detained in other states, in their own state, the law ensures that the same legal regime applies to MEPs as to their country's national parliamentarians."
Which means that Puigdemont's MEP immunity would not protect him in Spain.
"Puigdemont runs a serious risk coming to Spain", Ruiz said. "If I were his lawyer, I would tell him not to try."
The former president has said that he wants to find out if "Spain is the European exception".
To Ruiz, there is no such exception: "European immunity is regulated by a 1979 act that dictates that a MEP has the same prerogatives as Spanish deputies and senators. A deputy can be arrested if there is already an arrest warrant."
If the European Union applies literally what is established in the law, there would be no reason for dispute between Europe and Spain, he said.
Were Puigdemont to enter Spain and be arrested, the Catalan leader would probably file an appeal before the European Court of Justice, Ruiz said.
Spain would then have to await the court's decision, and lawyers of the European Parliament could be consulted too.
The question, he said, is how Spain will choose to apply the law.