According to a report in the Telegraph, the bank is being investigated over possible violations of the US Bank Secrecy Act. It has said it is co-operating with another investigation by the Department of Justice and the Office of Foreign Asset Control (OFAC) "regarding historical transactions involving Iranian parties and other parties subject to OFAC sanctions".

In its annual filing with the Securities and Exchange Commission, the bank said "it is likely there will be some form of formal enforcement action which may be criminal or civil in nature in respect of some or all of the ongoing investigations."

It added that fines and penalties resulting from the investigations could be "significant".

HSBC, which this week announced profits of £11bn for 2011, also said the investigations could result in a deferred prosecution in which the accused admits wrongdoing and leaves open the possibility of future prosecution if violations continue.