Criminals could be laundering their money through cryptocurrency cash machines, the Metropolitan Police has said.
Scotland Yard has warned that illegal cash could be deposited into the machines - which converts the money into digital currency such as Bitcoin.
Detective Superintendent Nick Stevens, of the Met's Serious and Organised Crime Command, said they have identified the use of cryptocurrency in organised criminality and are taking action to combat the threat.
"Cryptocurrencies are not illegal, but they are unregulated, decentralised currencies that can be quickly transferred across borders and then converted into the currency of the country where the funds are received," he said.
"Organised criminal groups have been early adopters of cryptocurrencies to evade traditional money laundering checks and statutory regulations.
"Criminals have also used cryptocurrencies to purchase illegal commodities on dark market sites with anonymity."
It is believed there are 91 cryptocurrency ATMs across the UK, the Met said.
The force said they are aware of the introduction of these ATMs - common in convenience stores and newsagents - which could be used by criminals to launder the proceeds of crime.
The machines allow cash to be moved around without being traced, including across borders, and without any checks.
Dep Sup Stevens said the Met is investigating several organised crime groups that use cryptocurrencies to run their activities across a range of crime types including drugs, firearms, modern slavery and child exploitation.
He said the force is also training officers about cryptocurrencies, particularly those involved in forensic examination of digital devices, so they are able to spot how they can be used.
Bitcoin is the world's leading cryptocurrency and uses a decentralised technology - or blockchain - for secure payments and works without the need for a central bank.
Coins are transferred directly from person to person, and are kept in a digital wallet that can be accessed on a computer or mobile device.
A crackdown is also being mounted by the Government on Bitcoin, amid rising concerns about money laundering and tax evasion.
The Treasury has said it intends to regulate the digital currency to bring it in line with anti-money laundering and counter-terrorism financial legislation.
Under the EU-wide plan, online platforms where Bitcoins are traded will be required to carry out due diligence on customers and report suspicious transactions.
The move is designed to end the anonymity which has made the currency attractive to criminals.