US authorities have reopened investigations into British bank Standard Chartered's breaking of sanctions on Iran, sending the company's share price tumbling, media reports said.

The London-based firm has already been fined almost £1 billion after it was found to have helped hide or disguise the identity of Iranian clients in billions of dollars worth of wire transfers.

But the U.S. Justice Department, Manhattan District Attorney Cyrus Vance Jr. and Benjamin Lawsky, superintendent of New York's Department of Financial Services, are all reopening their original inquiries into the lender to determine whether it intentionally withheld information from regulators before the 2012 settlements.

According to a report in the New York Times, regulators investigating another bank uncovered evidence of transactions that Standard Chartered had not disclosed.

Standard Chartered shares fell 4.9 per cent in London on Thursday. The stock has dropped about 31 per cent this year, the worst performance among the five largest British lenders.

The lender, which makes about three-quarters of its earnings in Asia, this week posted a 16 per cent drop in third- quarter pretax profit to US$1.53 billion from a year ago as impairments for bad loans almost doubled and regulatory and compliance costs increased. Underlying profit for the second half will decline from a year earlier, the bank said on Oct. 29.