The bill has proved highly controversial, since it does not encompass gambling on lotteries or horse racing, which are increasingly valuable sources of tax revenue in many states.

The bill is expected to be made more stringent - with the inclusion of criminal penalties - once it comes before the Senate.

The Senate Banking and Finance Committee has held hearings on online gaming, but has not yet scheduled a vote on this bill.

Online worries

The bill is the fruit of lengthy Congressional discussions over online gaming.

Most forms of online gambling are technically illegal in the US, but the law can do little to touch offshore operations.

These casinos, which number more than 1,500, have combined US revenues of more than $2bn.

Legislators have been wary of rewriting gambling law, which is notoriously complex, operating at federal, state, and in some places tribal level.

The current bill seeks instead to clamp down on credit-card payments and other cash transfers, something that can be done with the cooperation of financial services companies.

Playing politics

The bill is also seen as a shot in the arm for state-approved forms of gambling.

This could even allow online sports bookmakers to extend their operations into states such as Utah, where all gambling is currently banned.

At the same time, this has caused controversy in Congress, and an amendment to remove such preferences was narrowly defeated.

The bill's supporters have had to tread a careful line to keep it on track, and have come close to withdrawing it owing to lack of potential support.