A growing number of German banks are passing on negative interest rates to their retail customers as the costs become too high to bear on their own. Berliner Volksbank, the country’s second-largest cooperative lender, started to apply a minus 0.5% rate on deposits exceeding 100,000 euros.

With rates now well below zero, even Denmark’s $440 billion pension system says the environment has become so punishing that it may be time for a change in European rules. The warning comes as pension firms across Europe struggle to generate the returns they need to cover their growing obligations.