Watchdog the Advertising Standards Authority (ASA) banned ads for cryptocurrency trading firm Luno that appeared on the London Underground and on buses in February.
The ads stated: 'If you're seeing Bitcoin on the Underground, it's time to buy.' But there was no mention of the risk involved.
Cryptocurrencies are not regulated by the Financial Conduct Authority (FCA), so buyers have no protection under the Financial Services Compensation Scheme and cannot complain to the Financial Ombudsman Service.
The FCA has warned that investors betting on cryptocurrencies should could lose all their money.
H ong Kong plans to restrict cryptocurrency trading to professional investors only.