Standard Chartered's shares fell over 20% at one stage today after New York's top bank regulator threatened to remove its state banking licence.
The US regulator said the British lender hid $250 billion in transactions tied to Iran.
The New York State Department of Financial Services (DFS) said yesterday that Standard Chartered "schemed" with the Iranian government.
The DFS said the bank hid some 60,000 secret transactions to generate hundreds of millions of dollars in fees over nearly 10 years.
The British bank, which the regulator called a "rogue institution" also exposed the US banking system to terrorists, drug traffickers and corrupt states, the DFS said.
London based Standard Chartered said it was surprised by the statement, even though it has been in talks with US regulators over the matter for years.
The regulator's move is a savage blow to Standard Chartered, which has been one of the banks least tarnished during the financial crisis thanks to its focus on Asia and other emerging markets and a conservative capital and liquidity approach.
Standard Chartered said the bank "does not believe the order issued by the DFS presents a full and accurate picture of the facts."
The loss of a New York banking licence would be a devastating blow for a foreign bank, effectively cutting off direct access to the US bank market. Standard Chartered processes $190 billion every day for global clients, the New York bank regulator said.
In an unusual look inside a bank, the New York regulator described how Standard Chartered officials debated whether to continue Iranian dealings.
In October 2006, the top official for business in the Americas, whom the regulator did not name, warned in a "panicked message" that the Iranian dealings could cause "catastrophic reputational damage" and "serious criminal liability."
A top executive in London shot back: "You ... Americans. Who are you to tell us, the rest of the world, that we're not going to deal with Iranians." The reply showed "obvious contempt for US banking regulations," the regulator said.
Standard Chartered is the third British bank to be ensnared in US law enforcement probes this summer. Barclays agreed to pay $453m to settle US and UK probes that it rigged a global lending benchmark in June.
A month later, a US Senate panel issued a scathing report that criticized HSBC Holding Plc's efforts to police suspect transactions, including Mexican drug traffickers.
Standard Chartered said it shared with US agencies an analysis that demonstrated it "acted to comply, and overwhelmingly did comply" with US regulations. It put the total value of Iran-related transactions that did not follow regulations at less than $14m.
"The group was therefore surprised to receive the order from the DFS, given that discussions with the agencies were ongoing," Standard Chartered said. "We intend to discuss these matters with the DFS and to contest their position," it added.
Standard Chartered, a financier in emerging markets, is the sixth foreign bank since 2008 to be implicated in dealings with sanctioned countries such as Iran in investigations led by federal and New York law enforcement officials.
Four banks - Barclays, Lloyds, Credit Suisse Group and ING Bank - have agreed to fines and settlements totaling $1.8 billion. HSBC currently is under investigation by US law enforcement, according to bank regulatory filings.
The New York regulator, headed by former prosecutor Benjamin Lawsky, ordered Standard Chartered to explain why the bank should not lose its state license and the ability to process dollar transactions. Lawsky also ordered the bank to bring in an outside consultant to monitor its transactions.
"Standard Chartered Bank operated as a rogue institution," Mr Lawsky said in the order.
