The Financial Conduct Authority (FCA) and Serious Fraud Office (SFO) have been asked to investigate whether banks HSBC and Standard Chartered may "inadvertently have been conduits" for money laundering in South Africa.
Labour peer Peter Hain has written a letter to the chancellor Philip Hammond, which the Treasury has passed onto the financial regulators, asking them to look into whether the two banks were involved in money laundering.
Hain told the BBC a whistleblower had indicated they "maybe inadvertently have been conduits for the corrupt proceeds of money".
The peer is due to raise the issue in the House of Lords later today.
A Treasury spokesman said: "We take allegations of financial misconduct very seriously, and have passed Lord Hain's letter on to the Financial Conduct Authority and relevant UK law enforcement agencies, including the National Crime Agency and Serious Fraud Office, to agree the right action."
HSBC declined to comment on the matter.
A Standard Chartered spokeswoman said: "We are not able to comment on the details of client transactions but can confirm that following an internal investigation accounts were closed by us in 2014."