Bitcoin is "more of a speculative asset than money" that should perhaps "be seen more like a community of online gamers, who exchange real money for items that only exist in cyber space," says Carstens in a speech on digital currencies to the Hoover Institute.

More importantly, "investors must be cognisant that Bitcoin may well breakdown altogether" as it approaches its maximum supply of 21 million coins and faces up to a 51% attack.

The central bank's central banker says that Diem - previously called Libra - is "certainly more credible than Bitcoin" but adds that "overall, private stablecoins cannot serve as the basis for a sound monetary system".

These stablecoins may have some specific use cases but need to be heavily regulated and supervised and to be built on the "foundations and trust provided by existing central banks".

Ultimately, according to Carstens, "if digital currencies are needed, central banks should be the ones to issue them".

A new BIS survey shows that 86% of 65 central bank respondents are doing some kind of CBDC research or experimentation, with some - notably China - well on the way to issuing a digital currency.

CBDCs could play a catalytic role in innovation, spurring competition and efficiency in payments, says Carstens, but they come with a host of technological, legal and economic issues.

A recent paper from the IMF suggested that close to 80 percent of the world’s central banks are either not allowed to issue a digital currency under their existing laws, or the legal framework is not clear

"In all this, the need for international coordination cannot be overstated. It is up to individual jurisdictions to decide whether they issue CBDCs or not. But if they do, issues such as 'digital dollarisation' and the potential role of CBDCs in enhancing cross-border payments need to be addressed in multilateral forums," concludes Carstens.