Several firms have said they could stop taking bets from US customers if the bill is signed into law by President George W Bush in the next two weeks. The law, part of a 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.

Most analysts expected there would not be enough time for US Congress to pass a specific law against online gambling before next month's mid-term elections. Shares in Partygaming fell by nearly 60% in London trade, while shares in 888 Holdings tumbled by more than 45%. Sportingbet shares fell by nearly 60% while Empire Online shares slid 27%.

It was unclear whether the bill included poker, which resides in a legal grey area on whether it is a game of skill or chance. Analysts also noted that the bill potentially exempts certain financial instruments if it is not reasonably practical for transactions to be blocked, which may give the industry a back door.