Germany's constitutional court has rejected a challenge to the European Central Bank bond-buying program credited with helping keep the euro currency union from breaking up in 2012.

The court in Karlsruhe said Tuesday that the ECB had not exceeded its legal powers and rejected a challenge from legislators and a citizen's group to the bond-buying program.

ECB President Mario Draghi announced the program in 2012 after promising that the central bank for the 19 countries that use the euro would do "whatever it takes" to preserve the euro. At the time, bond market turmoil was threatening the finances of several euro countries. The ECB offered to buy bonds of countries facing high sovereign borrowing costs, as long as they submitted to financial reforms.

The program was never used, but the mere existence of an unlimited monetary backstop from the central bank helped lower the borrowing rates for governments struggling with excessive debt, easing the financial crisis. The court decision is a victory for the ECB and its ability to intervene in financial markets on behalf of its interest-rate policy, and a defeat for skeptics who think its stimulus and anti-crisis programs have gone too far.

Opponents had argued that the bond purchase offer violated the European Union treaty's ban on the central bank using its powers to print money to finance governments. The German court had passed the decision to the European Court of Justice, which last year upheld the bond purchase program but imposed conditions, and then returned the case to the German court.