The Bank of England's deputy governor for prudential regulation is warning banks about exposure to crypto assets.

In a new letter published Thursday, Sam Woods, chief executive of the Prudential Regulation Authority (PRA) – a financial services regulator in the U.K. – wrote that banks, insurance companies and investment firms should take steps to protect themselves against market volatility and potentially risky investments in the crypto space.

The deputy governor's warning focused strictly on the tokens themselves, reminding financial institutions that they have a fiduciary responsibility under PRA regulations.

Woods wrote:

The letter further stated that financial institutions should take steps to minimize any possible risk caused by trading in crypto assets, including having a PRA-approved Senior (Insurance) Management Function auditor review and authorize risk assessment frameworks for dealing with the new class.

Firms must also avoid excessive risk-taking, ensure access to experts in crypto assets and conduct due diligence on any assets they may want to trade in.

"Classification of crypto-asset exposures for prudential purposes should reflect firms' comprehensive assessment of the risks involved," Woods wrote, adding that these classifications should cite potential risks when investing in cryptocurrencies.

The deputy governor also pushed back against the idea that cryptocurrencies are a form of money, saying "crypto-assets should not be considered as a currency for prudential purposes."

In the letter, Woods also took the time to acknowledge that distributed ledgers and blockchain technology exist separately from cryptocurrencies, writing, "We also recognise that the underlying distributed ledger or cryptographic technologies, on which many crypto-assets rely, have significant potential to benefit the efficiency and resilience of the financial system over time."

Bank of England image via Shutterstock