Spanish lawmakers in May narrowly approved the bill which grants an amnesty to hundreds of separatists involved in Catalonia’s botched 2017 independence bid that triggered Spain’s worst political crisis in decades.

Socialist Prime Minister Pedro Sánchez’s minority coalition government depends on Catalan separatist parties to pass legislation and any complication in the application of the law could cause them to withdraw their support.

READ ALSO: Spain’s contested Catalan amnesty bill comes into force

He agreed to the controversial measure in exchange for the separatist parties’ parliamentary support to secure a new term in office.

Spain’s conservative opposition has staged massive street protests against the amnesty law, which judges must decide to apply on a case-by-case basis.

The Constitutional Court, which has a conservative majority, said it had “unanimously” accepted a request from the Supreme Court to rule on the constitutionality of the law as it could violate the principle that all Spaniards are equal before the law.

The court could take months to review the amnesty law. Dozens of people have already benefited from the measure, while others are still waiting for the courts to rule on their case.

Judges have refused to apply the measure to several high-profile Catalan pro-independence figures, including Puigdemont who fled Spain shortly after the failed 2017 secession bid to avoid prosecution and currently lives in Belgium.

Spain’s main opposition Popular Party (PP) and several regional governments, including one led by the Socialists, have also filed a challenge against the amnesty law with the Constitutional Court.

READ ALSO: Fugitive separatist Puigdemont returns to Spain but vanishes again