Cryptocurrencies are not scalable and are more likely to suffer a breakdown in trust and efficiency the greater the number of people using them, the Bank of International Settlements said on Sunday in its latest warning about the rise of virtual currencies.

For any form of money to work across large networks it requires trust in the stability of its value and in its ability to scale efficiently, the BIS, an umbrella group for the world's central banks, said in its annual report.

But trust can disappear instantly because of the fragility of the decentralized networks on which cryptocurrencies depend, the BIS said.

Those networks are also prone to congestion the bigger they become, according to the BIS, which noted the high transaction fees of the best-known digital currency, bitcoin, and the limited number of transactions per second they can handle.

"Trust can evaporate at any time because of the fragility of the decentralized consensus through which transactions are recorded," the Switzerland-based group said in its report.

"Not only does this call into question the finality of individual payments, it also means that a cryptocurrency can simply stop functioning, resulting in a complete loss of value."

The BIS' head of research, Hyun Song Shin, said sovereign money had value because it had users, but many people holding cryptocurrencies did so often purely for speculative purposes.

"Without users, it would simply be a worthless token. That's true whether it's a piece of paper with a face on it, or a digital token," he said, comparing virtual coins to baseball cards or Tamagotchi.