“They are not backed by governments or central banks,” states an advertisement on London’s Tube trains for the cryptocurrency trader Etoro. “Cryptocurrencies are backed by technology and trust.” In other words, if you think that your money is not safe in financial institutions, you can stash it away from their prying eyes. There is also an implicit promise of large returns.

Yet the message is at odds with the government’s stance on cryptocurrencies, of which there are about 1,300, with a worth of £500 billion. Mark Carney, the governor of the Bank of England, last week warned that cryptocurrencies were exhibiting the “classic hallmarks of bubbles” and demanded regulation to stop money being lost.

What are cryptocurrencies?

They are alternative digital currency systems that originated