Standard Chartered PLC’s stock price fell the most in almost 24 years, as an analyst estimated it may face costs of $5.5 billion after being accused of violating U.S. money laundering laws over its dealings with Iranian banks, Bloomberg reported. 

London-based Standard Chartered today denied the allegations, saying it “strongly rejects the position and portrayal of facts” made by the regulator. Its shares fell 23 percent to 1,132 pence by 10:42 a.m. in London trading, their biggest decline since 1988, the earliest date for which data are available.

Standard Chartered may lose its license to operate in New York after the state’s Department of Financial Services found the bank conducted $250 billion of deals with Iranian banks over seven years and earned hundreds of millions of dollars in fees for handling transactions for institutions subject to U.S. economic sanctions.

The London-based lender today denied the allegations, saying it “strongly rejects the position and portrayal of facts” made by the regulator.

The bank may be fined $1.5 billion by U.S. regulators, lose about $1 billion of revenue from its Iranian operation and a further $3 billion in market value if senior managers quit, Cormac Leech, an analyst at London-based Liberum Capital Ltd. who rates the stock a buy, wrote in a note to investors today.

“It’s unclear whether senior management will resign for the alleged shortcomings given that they have been in their current roles for much of the relevant period, raising the risk of kitchen-sinking on arrival of new management,” Leech said.

Though Standard Chartered doesn’t have any domestic U.S. banking operations, the loss of the New York license would hinder its ability to process dollar payments for clients with businesses in the U.S. and in emerging markets, said Gary Greenwood, an analyst at Shore Capital in Liverpool, who rates the stock a buy.