The European Union's top court has ruled that Catalan separatist leader Oriol Junqueras was covered by immunity as an MEP when he was jailed by the Spanish Supreme Court in October, prompting calls for his immediate release.

Following the advice released last month by Advocate General, Maciej Szpunar, the European Court of Justice said that Junqueras became an MEP from the moment the European election results were announced at the end of May and that he thus "enjoys the immunity guaranteed by Article 9 of the Protocol".

The ECJ also emphasised that Junqueras also enjoys "the immunity as regards travel" which applies "while they are travelling to the place of meeting of the European Parliament, including to that first sitting".

Junqueras, the former deputy leader of Catalonia, was sentenced to 13 years in jail in October, alongside a number of other Catalan separatist leaders, over the 2017 independence referendum and subsequent declaration of independence ruled illegal by Spanish courts.

Junqueras, who had been in pre-trial detention since 2017 was elected to the EU Parliament in May for the Greens/European Free Alliance group but Spanish courts refused to allow him to take an oath on the country's constitution, which they say is necessary to become an MEP.

The pro-independence Esquerra Republicana party, which Junqueras leads, has welcomed the ruling, calling for his sentencing by Spanish courts to be nullified and for his "immediate release".

What does the ECJ ruling mean for Catalan separatist leaders?

Ignacio Gonzalez Vega of Judges for Democracy, a Spanish legal association, told Euronews this development doesn't mean Junqueras will be released, nor does it necessarily mean a change to his sentence.

He said the ECJ ruling could not be applied retroactively as is already in jail for the crime of sedition.

"The big unknown" he said, is how the ruling could affect the situation of former Catalan president Carles Puigdemont and his ex-counselor Toni Comín, who are currently in self-imposed exile in Belgium to avoid prosecution by the Spanish authorities.

Both also won seats in the European elections in April and have several legal avenues open to them to claim their immunity.

González Vega believes they could return to Spain and the question would be whether they would enjoy the same privileges of immunity as Spanish MEPs.

Maciej Szpunar, the ECJ's Advocate General, said last month that the EU "parliamentary mandate may be acquired solely from the electorate and may not be conditional on the completion of any subsequent formality" including "taking an oath to respect the Spanish Constitution"

He also argued that "it falls to the (EU) Parliament to decide whether it is appropriate to waive or defend the immunity of one of its Members", suggesting Junqueras could have called on the EU parliament to defend his mandate.