Two US senators have written a letter to the US Attorney General alerting them of the existence of an online underground website called Silk Road facilitating exchange of illegal drugs.

The letter, whose copy was also sent to the Drugs Enforcement Agency, says that users of Silk Road can "freely purchase and sell illegal drugs . . . from cocaine and heroin, to ecstasy and marijuana".

The senators also said that the using an online currency called Bitcoin. The currency was created by an anonymous Japanese computer science student using the pseudonym Satoshi Nakamoto. It was first used by hackers for making exchanges and is now emerging as the de-facto unofficial currency for the internet.

Bitcoin can be purchased by anyone using major global currency and is also used for many other purposes. Several firms from legal services to landscape gardening, video game retailers to makers of alpaca socks are adopting Bitcoin.

According to estimates, one coin is valued at £9.22 and the whole Bitcoin economy is estimated at around £32.6m and growing fast. The currency does not have a single issuing authority and can be created by anyone with a powerful computer and a lot of time. It is only controlled using a series of complex mathematical algorithms built into the software.

The move by the senators indicates that authorities are taking note of the new currency and could face crackdown.