“The first we would know about it sometimes is when you crack open their safe and inside there’s a [digital] pen drive, containing loads of codes and IDs which you find are linked to crypto-assets,” adds Benton, who now runs consultancy Intelligent Sanctuary

“It is a massive issue. And quite often, the exchanges where the transactions take place are in jurisdictions where there is only very light regulation.”

Some crypto exchanges have been criticised for requiring almost zero verification of users wanting to carry out transactions, including a minority that let people act anonymously.

These concerns were echoed this week by US Federal Reserve chairman Jerome Powell.

“We have this burgeoning industry which has many parts to it, and there isn’t in place the kind of regulatory framework that needs to be there,” he told American lawmakers.

However, Changpeng Zhao, the boss of cryptocurrency exchange Binance, has argued digital currencies are no more susceptible to money laundering than physical assets such as cash. 

He rejected calls for a blanket ban on Russian purchases at exchanges, a measure called for by the Ukrainian government, saying this would only hurt ordinary people using the platform for legitimate purposes. Binance carries out “know your customer checks” and abides by sanctions, he added.

Zhao told the BBC: “If people want to evade sanctions, there’s always multiple methods. You can evade sanctions using US dollars, using cash, using diamonds, using gold. I don’t think crypto is anything special there.” 

There is some evidence that law enforcement is already adapting to the challenges posed by Bitcoin.

New York husband and wife Heather Morgan and Ilya Lichtenstein were recently arrested by US authorities in connection with one of the biggest cryptocurrency heists on record: the theft of $4.5bn worth of Bitcoin stolen from the Bitfinex exchange in 2016.

FBI investigators said they used “good old fashioned police work” to track a complex string of transactions back to the pair.