The hearing in Karlsruhe came as the ECB studies an expected new round of purchases that would add to the 2.6 trillion euro ($2.90 trillion) pile of assets the euro zone's central bank hoovered up between 2014 and last year.

But German constitutional court judges warned about the risks associated with the ECB's purchases of sovereign debt, which have driven down borrowing costs for governments across the euro area.

"These are losses on savings products, life insurances, pension products and a decrease of the pressure on states to consolidate their public finances," the president of the court Andreas Vosskuhle said at the hearing.

The constitutional court could potentially order the Bundesbank -- which as the central bank of the euro zone's biggest economy carries out the largest share of the ECB's bond buys -- not to take part in new purchases.

But the likelihood of that is low after the European Union's top court, the highest authority in cases against EU institutions, last year approved the ECB's previous bond-buying programme.

The ECB also took the EU ruling as giving it broad discretion when designing future stimulus programmes, meaning it is seen likely to relax the constraints that it put in place when it started buying government bonds in 2015.

Two German government advisers heard by the court on Wednesday, Lars Feld and Volker Wieland, also defended the ECB's stimulus programme.