The Spanish parliament suspended on Friday the lawmaker rights of the four jailed Catalan members of parliament while they face trial over the region's failed 2017 independence bid.
The ruling socialist party PSOE, the conservative Popular Party and the centrists of Ciudadanos voted in favour of the decision, while left-wing Podemos voted against.
The announcement was made by lower House speaker Meritxell Batet on behalf of the parliament's governing body.
Catalonia's regional vice president Oriol Junqueras, who is among the jailed MPs, immediately denounced the move.
"We have been suspended in Spain but they can not do this in Europe," Junqueras wrote on Twitter with reference to ongoing EU elections.
"A political prisoner entering the European Parliament is the best way to denounce the repression of the Spanish State," Junqueras continued.
It was not immediately clear what would happen to the seats as it's up to the four lawmakers to decide if they want to leave them empty for now or if they would rather resign and pass the position to a fellow politician.
If the seats are left empty, the total number of lawmakers would drop which, in a deeply fragmented parliament, may work in favour of acting Prime Minister Pedro Sanchez who could then be reelected without the backing of the Catalans.
Oriol Junqueras and fellow separatists Josep Rull, Jordi Turull and Jordi Sànchez won seats during Spain's April 2019 general election. Raül Romeva, meanwhile, won a seat in Spain's senate.
They are currently being held in prison, awaiting trial on rebellion, sedition and embezzlement charges. It is linked to their role in the 2017 Catalan independence referendum, which Madrid later declared illegal.
In an unprecedented move on Monday, five Catalan leaders were allowed out of prison on Tuesday to be sworn in at the opening of the Spanish Parliament.
But the Spanish Supreme Court recommended to the parliament's governing body to suspend the jailed MPs during their detention. Under Parliamentary rules, any member placed in temporary detention is automatically suspended.