LONDON (Reuters) - Some UK-based banks are dodging a ban on offering investment services to EU-based customers by targeting them individually through advertising, the bloc’s regulator said on Wednesday, warning of possible legal action over such “questionable” practices.

Britain left the European Union on Dec. 31 and their post-Brexit trade deal does not cover financial services, effectively cutting off the City of London from its biggest customer base.

Brussels has said it won’t consider access to investment services from UK-based investment banks for the time being, prompting them to seek workarounds.

Since Dec. 31, “some questionable practices by firms around reverse solicitation, where the product or service is marketed at the client’s own exclusive initiative, have emerged,” the European Securities and Markets Authority said in a statement.

Some banks were trying to circumvent EU requirements by including general clauses in the terms of business or via online pop-up “I agree” boxes, whereby customers say that any transaction is executed on the exclusive initiative of the client, the watchdog said.

ESMA said a bank outside the bloc that promoted its investment services in the EU could be deemed as a service provided at the own exclusive initiative of the customer, regardless of any contractual clause or disclaimer.

Provision of investment services in the EU without proper authorisation “exposes service providers to the risk of administrative or criminal proceedings”, it added.