It ruled that, having sworn the oath, Junqueras would then have to take possession of his European Parliament seat on July 2, and that is impossible since he is in custody as he is considered a flight risk.
Ex-Catalan president Carles Puigdemont, Junqueras' old boss, and other former members of his government fled Spain to avoid arrest and reside in Belgium. Junqueras is one of a dozen separatists on trial in Spain for charges that include rebellion for their role in the 2017 secession attempt led by Puigdemont's regional government in northeastern Catalonia. They face years in prison and being banned from holding public office. The public hearings of the trial concluded this week. A verdict is expected in the coming months.
In Friday's ruling, the Supreme Court said that if Junqueras was allowed to leave Spain to attend the European Parliament, it would mean "the loss of judicial control." It added that Brussels "is the place where one of fugitives from justice says that he has established the seat of the Catalan Republic's government in exile" in reference to Puigdemont's campaign to drum up international support for the separatists' cause.
The court said that its decision "does not mean the irreversible loss" of Junqueras' right to hold office, saying that once a verdict is issued he could then have the possibility of taking possession of his seat.
Junqueras was elected in May 26 elections along with Puigdemont and fellow fugitive separatist Toni Comín. Puigdemont and Comín would face arrest if they returned to Spain on Monday to attend the oath-swearing session at the Parliament.
Last month, Junqueras told The Associated Press in an interview via videoconference from jail that he would defend his rights "before all possible judicial institutions" if blocked from reaching the European legislature.