Government bond-buying is at the centre of a complex legal row, bringing national and federal justice into the spotlight.
A legal battle is brewing in the EU over a judgement earlier this week in Germany, and the EU's top court has fired back.
In a statement, the Head of the European Commission Ursula von der Leyen said the commission will now analyse the German court ruling and look into possible steps, which "could include the option of infringement proceedings."
She added, the "final word on EU law is always spoken in Luxembourg. Nowhere else."
Germany's Constitutional court ruling on May 5, which took aim at the European Central Bank's mass bond-buying scheme, had wider implications for the European Court of Justice (ECJ), the top legal authority of the bloc.
The ruling related to government debt worth €2.1 trillion purchased by the ECB starting in 2015 (so-called quantitive easing). The court said that there was not enough political oversight in the buying scheme.
The ECJ responded on Friday that they "consistently held that a judgment in which the Court gives a preliminary ruling is binding on the national court." Indeed, the ECJ ruled in 2018 that government bond purchasing was legal.
The press release went on to state that the court was the only authority with jurisdiction to rule whether or not an act of an EU institution is contrary to EU law. This was to 'ensure that EU law is applied uniformly'.
The release went further to state that divergences between EU country's courts 'would be liable to place in jeopardy the unity of the EU legal order'.
Germany's constitutional court said the ECJ had effectively overstepped its authority, accusing them of acting 'ultra vires' (beyond their powers) in the 2018 ruling.
However, the European Commission already reaffirmed the 'primacy of EU law' and the fact that all ECJ rulings are 'binding on all national courts'.
Concerns were raised over Germany's ruling in Brussels, that it could set a dangerous precedent if national courts could overrule the EU's top court.
Notably, Poland which is currently subject to legal scrutiny by the ECJ, celebrated the German ruling. Deputy Justice Minister Sebastian Kaleta, said that the Polish government had been arguing for months that "the EU cannot overstep its competences."