UK bookmaker William Hill has ceased taking casino and poker bets online from US customers.
William Hill wants to see US laws clarified
The firm said it would stop taking bets until the "scope and enforceability" of US state and federal gaming laws had been clarified.
It added that it already refused to accept online sports bets from people living in the US.
William Hill's move comes as US authorities have been clamping down on internet gaming firms.
On Tuesday, the chairman of UK-listed online gambling firm World Gaming, James Grossman, quit as fears grew that top executives at internet gaming firms could be arrested in the US.
The former chairman of Sportingbet and chief executive of Betonsports have both been arrested recently amid allegations of breaking US gaming laws.
Returns from online gaming in the US account for a small fraction of William Hill's overall business.
Chief executive David Harding said the firm had been advised that its activities were legal but believed that stopping US business was the "prudent course of action" given the uncertainty about the legal position.
"It is not a big deal," he said. "We are a bookmaker and we were always clear that sports betting was illegal."
Mr Harding added that as William Hill did not promote its business in the US, it had not been worried that any employees may face arrest.
Sports betting is illegal in most US jurisdictions but it is permitted in Nevada and many people still gamble.
William Hill's shares fell marginally after the firm's announcement, closing down 0.3% at 644 pence.
Betonsports' David Carruthers was arrested in July and is facing charges of racketeering, fraud, tax evasion and conspiracy as part of a wider crackdown on online gaming firms.
The charges stem from accusations that the firm failed to pay US excise taxes on more than $3bn in bets taken from US gamblers.
Mr Carruthers has pleaded his innocence but he has since been sacked and Betonsports has shut down its US business.
Sportingbet's Peter Dicks was detained earlier this month by the Louisiana authorities under a warrant of "gambling by computer".
He has been bailed to appear at a hearing in New York on Thursday, at which the authorities will seek his extradition to Louisiana.
Analysts say the legal actions are part of an attempt by US authorities to make online gambling illegal under the 1961 Wire Act.