After the European Central Bank's most recent rate cut in March, private-sector banks are paying what amounts to an annual levy of 0.4 per cent on most of the funds they keep at the eurozone's 19 national central banks.

This policy, which has cost banks around €2.64bn since ECB rates became negative in 2014, is intended to spark economic growth by incentivising banks to lend money out to businesses instead of holding on to it.

European central bankers say they could cut rates again should economic conditions worsen, but private bankers and insurers are already thinking of creative ways to avoid those charges altogether.

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One way is by turning the electronic money they keep at central banks into cold, hard cash. Munich Re has experimented successfully with storing a double-digit million sum of euros in cash at what the insurer describes as a manageable cost. A few other German banks, including Commerzbank, the country's second-biggest lender, have also considered taking the step.

But when a Swiss pension fund attempted to withdraw a large sum of money from its bank in order to store it in a vault, the bank refused to provide the cash, according to local media reports.

If this practice becomes widespread, it would have big economic implications. If banks are not paying central bank interest charges, then they won't be as affected by further official interest rate cuts. They therefore would not be spurred to lend out more money.