The idea of keeping piles of cash in high security vaults may sound like something from 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 heavily 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.

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 giving banks the incentive to lend money out to businesses instead of holding on to it.