Germany's top court cast doubt on the legality of the European Central Bank's (ECB) eurozone rescue plan.

The Constitutional Court took the rare step of asking the EU court for its opinion on the outright monetary transactions programme, which allows the ECB to buy government bonds from ailing eurozone members as an emergency measure.

Though the programme has never been used, its existence has been credited with easing the turmoil in bond markets that threatened the 18-member currency bloc's stability.

The ECB offer to buy bonds has resulted in a sharp drop in borrowing rates in Spain and Italy, calming fears from speculators that the countries might default and drop out of the euro.

German opponents of the measure, however, claim that such bond purchases could fuel inflation and amount to monetary financing of governments - forbidden under the ECB mandate.

The German court indicated the argument might have merit.

Explaining why it was referring the case to the European Court of Justice, Germany's Constitutional Court said there were "important reasons to assume" that the ECB bond-buying programme exceeded the bank's mandate and "thus infringes the powers of the member states."

But the Luxembourg-based justice court is seen by some analysts as more favourable to the ECB and the German court's referral could end up working in the bank's favour.

The ECB merely said that it "took note" of the decision.