Germany's Constitutional Court has ruled that the country's central bank must stop participating in a key European Central Bank stimulus program but gave the ECB time to demonstrate that the stimulus program is needed and appropriate.

The judges of the Karlsruhe-based court ruled Tuesday that Germany's central bank, the Bundesbank, must stop buying bonds as part of an ECB stimulus program begun in 2015 unless the ECB reaches a “new decision” on the program that demonstrates its effects on the economy were “proportionate." It also said the Bundesbank should sell the bonds, but only in accord with the ECB and over the long term.

The ECB has bought more than 2.6 trillion euros ($2.9 trillion) in corporate and government bonds in an attempt to raise inflation and weak economic growth in the 19 countries that use the euro. The program faced objections from conservative German academics that it exceeds the bank's authority and violates a legal ban in the EU treaty on financing governments. The court found that the ECB did not violate a ban on central banks financing government spending.

The court said that “the Bundesbank is barred, after a transitional period of at most three months, from participating in the implementation and carrying out of the decisions at issue, unless the ECB council demonstrates in a new decision that the goals sought by the purchase program are not disproportionate to their economic and budgetary effects."