Online gaming firm Sportingbet has seen its share price plunge by 40% following the arrest of its chairman Peter Dicks.
The US is a major market for Sportingbet
Mr Dicks was held in New York on Thursday under a warrant issued by state authorities in Louisiana accusing him of "gambling by computer".
Shares in London-listed Sportingbet were suspended after the arrest, but when they resumed trading on Monday they sank by 39% to 145.75 pence.
On Friday, Mr Dicks was granted bail set at $50,000 by a US court.
Bail was granted on condition that he surrender his passport and remain within the five boroughs of New York.
Mr Dicks' detention comes two months after David Carruthers, the boss of rival firm Betonsports, was arrested on racketeering charges at Dallas airport.
The moves are being seen as part of a clampdown to discourage internet gambling in the US.
Mr Dicks is set to face another hearing on 14 September, when the New York court will consider a request by the Louisiana authorities to extradite him.
In a statement to the London Stock Exchange on Monday, Sportingbet said: "Neither Mr Dicks nor the Sportingbet Group has ever received any previous correspondence from any authority within the State of Louisiana regarding this or any other related matter.
"The board believes that Mr Dicks intends to vigorously contest this request."
Sportingbet said it was "closely monitoring the situation and continues to operate as normal". Last week it said it would continue to take bets from customers in the US.
The company, which also owns the Paradise Poker brand, has more than 2.5 million registered customers and an annual turnover of more than $1.2bn (£630m). About 70% of its profits are generated in the US.