Cynthia Haines used her credit cards to gamble over the Internet and was sued by her credit issuer, Providian National Bank, over unpaid bills. She then filed a countersuit against the bank as well as Visa and MasterCard, claiming the companies were trying to collect on gambling debts that were unenforceable under California law.

Haines's lawyer, Ira Rothken, said Haines sued MasterCard for injunctive relief, to "get them to stop violating California law".

Under the settlement, to continue using MasterCard credit cards, gambling sites will have to post a notice explaining that Internet gambling may be illegal in the user's jurisdiction, Rothken said. Internet casinos in their user agreements also must ask where the user is located.

"When you couple those requirements with the California law that stands, internet gambling sites would be taking the bets at their own peril," Rothken said.