"Our critical appraisal focuses on aspects of such liquidity provision where the boundary between monetary policy and fiscal policy is in danger of becoming blurred," said Weidmann, who also sits on the ECB's Governing Council.

"For example, when emergency liquidity assistance is granted on a very large scale, the collateral framework is severely watered down, or when government bonds are purchased," he added.

(Reporting by Balazs Koranyi; Editing by Maria Sheahan)