The US government is tipped to propose a system to identify and stop payment to gambling sites, say specialists.
US politicians are worried about gambling and consumer debt
The news comes after the US Congress passed a bill cracking down on internet gambling, sending shares in UK gambling sites plummeting on Monday.
By Monday close, Partygaming shares fell 58%, 888 Holdings tumbled by 26%, and Sportingbet shares fell 64%.
Some UK firms are looking at stopping bet-taking from US customers if the bill is signed into law.
It is expected that US President George W Bush could sign the bill into law within the next two weeks.
Not that bad
The law, part of a sustained clampdown on online gaming in the US, would make it illegal for banks and credit card firms to process online gaming payments from the US.
Following the new law, the Treasury Department and Federal Reserve have nine months to suggest new rules.
A so called "coding-and-blocking system" that would prevent payment to gambling sites is being considered, industry experts said.
"If the Treasury and Fed can come up with reasonable rules here, it should not be that bad" said lawyer Oliver Ireland, who has experience working with financial payment firms.
The news comes after firms in the online gambling arena have faced a turbulent year as the threat of a US legal crackdown has depressed share prices.
Bosses from two firms, Sportingbet and Betonsports, were arrested on illegal gambling charges.
Most analysts expected there would not be enough time for Congress to pass a specific law against online gambling before next month's mid-term elections.
But lawmakers took the industry by surprise when an "Unlawful Internet Gambling Enforcement Act" was eventually attached to an unrelated bill aimed at improving port security.
President Bush is thought unlikely to veto a bill which a senior Republican said was needed to regulate a "shadow industry".
Partygaming, 75% of whose revenues derive from the US, said the development was a "significant setback" for it and the whole industry.
One of the other firms affected said the move had come as a surprise.
"It has a specific criminal offence for taking these funds which is something completely new," Gigi Levy, chief operating officer of 888 Holdings, told the BBC.
"It is quite clear that the bill as is has a very clear implication on the legality of our activities in the US."
Mr Levy said his company was less dependent on the US market than it had been a year ago, having promoted itself in other countries.
"I can understand investors will feel a bit of panic," he said, while adding that he was "quite confident" that the firm would recover.
Gaming firms which have floated their shares over the past 18 months warned investors that their prospects could be affected by legal uncertainties in the US.
Analysts were divided over the extent of the likely damage to the online gaming business and its global ambitions.
One described it as "major shock" to the industry while another said it would remove the uncertainty hanging over the sector.
"I think there is still a lot of non-US business there and it is not as if these companies are going to disappear," said Richard Carter, an analyst at Numis Securities.
Meanwhile the decision to outlaw gambling elicited some strong support from US politicians.
"Gambling is a serious addiction that undermines the family, dashes dreams and frays the fabric of society," said Bill Frist, Senate Majority Leader.