The long-awaited advisory opinion is likely to play a key role in Socialist Prime Minister Pedro Sanchez's efforts to form a government with the support of Junqueras' party after months of political turmoil.
Junqueras won a seat in the European Parliament in May elections, despite being in pre-trial detention in Spain on charges linked to his role in organising a banned Catalan independence referendum.
But he was prevented from leaving prison in June to attend a Spanish swearing-in ceremony, and thus from travelling to Brussels to take up his post, prompting him to file an appeal.
He was sentenced in October to 13 years in jail.
The picture was further complicated on December 19 when the European Court of Justice ruled that Spain should have let him out to receive his credentials after the May elections, saying that even jailed MEPs benefitted from parliamentary immunity.
In the State Attorney advisory opinion, Spain's Supreme Court should accept Junqueras' appeal, saying it “could allow” him to travel to the central election commission in Madrid and the European Parliament headquarters to carry out the necessary formalities.
But at the same time, the state attorney also urged the court to ask the European Parliament “as soon as possible” to suspend his parliamentary immunity.
It also asked the court to let Junqueras carry out his representative functions as long as he held office, and to ensure “appropriate measures are in place to ensure his mission as an MP while serving the prison sentence” he was handed in October.
The Luxembourg-based court's ruling appeared to invalidate the Supreme Court's decision to prevent Junqueras from being freed to pick up his accreditation, but the full consequences stemming from the decision remain unclear.
The Supreme Court now has to issue its own ruling on the appeal filed by the former Catalan vice-president's defence team.