Spain’s Supreme Court says the man nominated to lead Catalonia’s regional government cannot be freed from jail to attend an investiture ceremony.
Pro-independence activist Jordi Sanchez, 53, had asked to be released to attend Monday’s debate and vote in the Catalan parliament.
He was jailed on sedition charges after October’s independence referendum, which was deemed illegal by Spain.
The judge said there was a risk Mr Sanchez would reoffend.
Mr Sanchez was nominated as Catalonia’s leader by separatist parties after the former head, Carles Puigdemont, last week formally abandoned his attempt to be reappointed. He has backed Mr Sanchez, who is the only candidate.
Mr Puigdemont fled to Belgium after the referendum to avoid detention. He remains there, fearing arrest if he returns.
The separatist parties won a slim majority in regional elections in December.
Those elections had been called by the central government after it sacked the regional administration and imposed direct rule when the independence referendum was declared illegal.
Judge Pablo Llarena ruled on Friday that although Mr Sanchez had the right to participate in public affairs, those rights could be curtailed if there was a risk of reoffending.
It remains unclear what will happen to Monday’s planned investiture debate.
Mr Puigdemont refused to rule out new elections if Mr Sanchez could not be sworn in.
“It is no tragedy if there are new elections, although it is not the priority and no-one desires it,” he told Catalan newspaper El Punt Avui.
The central government has vowed to oppose any attempt by Catalonia to break away and has told its parties to choose a leader who is not in jail.
Twenty-eight Catalan politicians are being investigated by a Supreme Court judge for their role in the referendum and a subsequent declaration of independence.
Mr Sanchez and three others remain in jail: former vice-president Oriol Junqueras, Catalan ex-Interior Minister Joaquim Forn and grassroots separatist leader Jordi Cuixart.
This is Spain’s biggest political crisis since democracy was restored to the country in 1975.