London-listed Standard Chartered has rejected a US regulator's claim that it hid \$250bn worth of illegal transactions with Iranian banks over nearly a decade.
The New York department of financial services says the bank operated as a "rogue institution", engaged in "deceptive and fraudulent misconduct", and lied to regulators to cover up its actions.
But the UK institution has hit back saying it "does not believe the order issued by the DFS presents a full and accurate picture of the facts".
It said in a statement: "The analysis, that the Group shared with all the US agencies, demonstrates that throughout the period the Group acted to comply, and overwhelmingly did comply, with US sanctions and the regulations relating to U-turn payments.
"As we have disclosed to the authorities, well over 99.9% of the transactions relating to Iran complied with the U-turn regulations.
"The total value of transactions which did not follow the U-turn was under \$14m."
The bank's Hong Kong-listed shares plunged 7.5% to HK\$174.0 when the market opened following the news.
Under US law, any financial dealings with Iranian banks are subject to sanctions because of concerns that US institutions could be used to finance the country's nuclear programme or terrorist operations.
The New York regulator alleges that the bank arranged to help the Iranians by omitting or falsifying the name of the client in order to ensure transactions did not raise any red flags.
The department says the bank engaged in prohibited U-turn transactions, where a non-US financial institution transfers funds to a US bank, which then wires money directly to another non-US financial institution.
It also alleges that the practice was sanctioned at the highest levels of Standard Chartered, quoting a bank director who said: "You f*****g Americans. Who are you to tell us, the rest of the world, that we're not going to deal with Iranians?"