The idea of keeping piles of cash in high security vaults across Europe may sound like something out an old movie plot, but some banks and insurers have recently started considering the idea as interest rates sink below zero across much of Europe.

Europe's highways are not yet jammed with guarded trucks transporting money to top secret locations, but if it becomes financially sensible for banks to hoard cash as rates are cut even further, the practice could undermine central banks' ability to use negative rates to boost growth.

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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. More from the Financial Times

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German court backs ECB crisis-fighting tool 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.

Fortunately for central banks, the hoarding of cash creates a host of other costs.

Part of it is storage and transport, though they are not the biggest problems. A withdrawal of a large amount of cash in one swoop would keep transport costs low, while the high value of the largest denominations of euro and Swiss franc notes mean that large amounts can be stored in small volumes. Even when the ECB stops issuing the €500 note in 2018 and banks will have to use the smaller denomination €200 notes, there is enough space in vaults, according to private bankers.

Bank robbers, earthquakes and other unforeseen disasters, on the other hand, are a problem. Or rather, the delicate issue of finding an insurer willing to take on those risks while charging a reasonable fee. "No one stores cash for large amounts of time. At the moment, cash comes in and then goes out quickly to ATMs," said a German banker, who has looked into the costs of switching to bank notes. The banker estimated that the insurance cost would probably be between 0.5 per cent to 1 per cent of the value of the banknotes being stored. That would be higher than the ECB's negative rate, but in line with Switzerland's minus 0.75 per cent, which is one of the lowest in the world.

