Washington, D.C. ? Placing bets over the Internet was effectively criminalized by the federal government Friday, as lawmakers work to eliminate an activity enjoyed by as many as 23 million Americans who wagered an estimated $6 billion last year.

Attached to a port security bill signed Friday by President Bush was the Unlawful Internet Gambling Enforcement Act, which prohibits online gamblers from using credit cards, checks and electronic fund transfers to place and settle bets. The law puts enforcement on the shoulders of banks and other U.S. financial institutions, some of which fought the legislation.

The new law is potentially crippling to a worldwide industry whose biggest customer has been the United States. Already, several online wagering businesses have pulled their operations out of the United States and some have collapsed, including publicly traded companies in Britain, where online betting is legal and regulated.