Online gambling is largely illegal in the United States but the law has proved to be difficult to enforce. The new measure would prohibit gamblers from using credit cards, cheques and electronic fund transfers to settle their online wagers.
888 Holdings, which operates a casino and poker website, said it had suspended online betting operations in the US causing shares to fall 48 per cent in London on Monday.
"The precise effect of the legislation is unclear," 888 Holdings said in a statement. "However, this legislation indicates congressional intent to treat Internet gaming, whether sports-related or not, as illegal."
PartyGaming announced separately that it would pull out of the US if George Bush, the president, signs the legislation into law. Its shares fell 60 per cent and Sportingbet shares were down 67 per cent.
The legislation, if enacted, "will make it practically impossible to provide US residents with access to real money poker and other real money gaming sites", PartyGaming said.
Port security bill
Republicans added the internet gambling measure to a bill aimed at improving port security, which was passed by the US House of Representatives and senate on Saturday
"The enforcement provision provided by this bill will go a long way to stop these illegal online operations," Jon Kyl, a Republican senator, said.
The measure's supporters include the National (American) Football League as well as conservative and anti-gambling groups. Some banking groups have lobbied against it.
Federal officials have made several recent arrests in connection to offshore companies operating internet gambling sites. The online gambling industry is based almost entirely outside the US but millions of Americans bet an estimated $6 billion a year online, according to industry figures.
The Wire Act of 1961 makes it illegal to make or accept bets on sporting events over the telephone and it is argued that since internet traffic travels over phone lines, the law also prohibits online gambling.