Germany's top court has asked an EU court to decide whether the European Central Bank's bond-buying program breaches EU law.

The referral to the European Court of Justice on Friday by Germany's Constitutional Court calls into question a key part of the ECB's plans to overcome the 18-nation eurozone's financial crisis.

The program allows the ECB to buy bonds of ailing eurozone members as an emergency measure in specific circumstances. Opponents claim this could fuel inflation and amounts to monetary financing of governments, which is forbidden by the ECB's mandate.

The so-called Outright Monetary Transactions program was key to avoid further increases in government bond yields for the weaker eurozone members such as Spain and Italy.

The ECB said in a statement that it "takes note" of the decision.