HSBC should be investigated for "possible criminal complicity" in connection with corruption allegations in South Africa, a UK peer has said.
Lord Peter Hain told Parliament he had asked the Treasury to refer an unnamed UK bank for investigation into possible involvement in money laundering.
In the letter, seen by the BBC, he named HSBC and said it ignored a warning about large transfers of cash.
He cited evidence of transactions out of South Africa involving HSBC.
The controversy relates to the close relationship between the wealthy Gupta family and South African President Jacob Zuma. The Guptas are alleged to have bought influence in government in order to loot state enterprises.
Mr Zuma and the Guptas strongly deny wrongdoing and say they are victims of a "politically motivated witch-hunt".
Lord Hain, a leading anti-apartheid campaigner who grew up in South Africa, has previously called for UK authorities to examine the role of British banks in the scandal.
Regulators are looking into HSBC and Standard Chartered's roles in South Africa following a separate letter from Lord Hain in September.
When asked about the issue earlier this week, HSBC's chief executive, Stuart Gulliver, said the bank was co-operating with the government.
Mr Gulliver said: "On the inquiries as a result of Lord Hain's letter to the chancellor, obviously we are responding to those inquiries that have come in from the [Financial Conduct Authority] and also from South African authorities, and there is nothing more I can really add at this point in time."
An HSBC spokesperson declined to comment further.
Lord Hain told the House of Lords on Wednesday that the latest evidence he had sent to the Treasury showed the illegal transfer of funds from South Africa made by the Guptas.
He said illicit transactions were "flagged internally in the bank concerned as suspicious, but I'm reliably informed they were told by the UK headquarters to ignore it".
"Can the chancellor please ensure that such evident money laundering and illegality is not tolerated and that the bank is investigated for a possible criminal complicity over this matter?" he said.
In September, Lord Hain requested that the Financial Conduct Authority, the Serious Fraud Office and the National Crime Agency look into whether British banks facilitated alleged money-laundering by the Gupta family.
At the time, he told the House of Lords that the banks may "inadvertently have been conduits" for up to £400m of illicit funds.
Standard Chartered said following Lord Hain's initial call: "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."