Commerzbank, one of Germany’s biggest lenders, is examining the possibility of hoarding billions of euros in vaults rather than paying a penalty charge for parking it with the European Central Bank, according to sources familiar with the matter.

Such a move by a bank part-owned by the German government would represent one of the most substantial protests yet against the ECB’s ultra-low rates, which have been criticised by politicians, including Finance Minister Wolfgang Schaeuble.

Although no decision has yet been taken, the lender has held discussions on the matter with German authorities, said two officials, who asked not to be named because of the sensitivity of the matter.

A spokesman for Commerzbank said it was not storing cash “at the moment” and declined to comment on whether it might do so in the future. The ECB declined to comment.

Commerzbank’s examination of storage alternatives to the ECB comes at a time of growing frustration among European lenders with the ECB charge on deposits.

Were it to store cash on a significant scale, it would become the first major European bank to take such a step. If other lenders were to follow suit, it could render the ECB penalty charge policy increasingly ineffective.

The ECB imposes a so-called negative rate equivalent to €4 annually on each €1,000 lenders deposit with the central bank. This is designed to encourage banks to lend money, rather than park it.

But some banks complain that a dim global economic outlook means there is weak demand for loans on the terms they require, and they havelittle option but to hoard cash.