A US jury has found a former HSBC executive from the UK guilty of defrauding Cairn Energy in a 2011 currency trade.
Prosecutors said Mark Johnson made millions at Cairn’s expense through a scheme to increase the price of sterling before a planned Cairn deal.
Mr Johnson, the former head of HSBC’s global foreign exchange cash trading, was arrested in New York last year.
The US is seeking to extradite another UK banker, Stuart Scott, in the case.
The lawsuit is related to a wider investigation into global currency markets.
While banks have admitted to conspiracy in currency trading, this suit was the first to target individuals.
Speaking to Reuters after the verdict on Monday, an attorney for Mr Johnson maintained his client’s innocence.
Prosecutors said Cairn, an oil and gas company listed in the UK, selected HSBC in 2011 to handle the conversion into sterling of $3.5bn in proceeds from the sale of one of its subsidiaries.
They alleged that Mr Johnson and Mr Scott, on behalf of HSBC, bought sterling in advance of the deal, inflating the currency’s value and ultimately making about $8m for the bank.
Mr Johnson was on trial for nearly four weeks in a federal court in Brooklyn, New York.
Mr Scott has denied the allegations.
HSBC, which was not charged in this case, declined to comment.
The firm has said it is in talks with US regulators to resolve an investigation of its foreign exchange trading.