The proliferation of cryptocurrencies, privately-issued stablecoins like Facebook’s Diem and non-fungible tokens could prove difficult for central banks to override with their own projects, former Bank of England governor Mark Carney has warned.

Central bank digital currencies are being explored by regulators in the UK, US, China and elsewhere, as banks attempt to innovate their monetary systems amid a boom in cryptocurrencies and other private solutions. The Bank of England’s effort is only in its earliest proposal stages, while Beijing has already begun consumer trials.