Supermarket's vouchers were popular item in illicit sales on 'dark net' site better known for dealing drugs and weapons

The dark net, the hidden network that can be accessed only using specialist equipment, has become a major distribution hub for illicit substances, weapons and images of child abuse. But now evidence has emerged that its most infamous incarnation, the Silk Road website, has been doing a roaring trade in a less exotic offering.

Screengrabs taken from Silk Road 2, the successor to the original site, reveal that its most popular items for sale until recently were Tesco Clubcard vouchers. Vouchers worth £100 each, offered by a vendor using the handle Revivalry, were selling for $61 (£43). On the same site, a vendor was offering premium Spotify accounts for $5.

Both offers on Silk Road 2, which have disappeared after apparently being subjected to a major fraud that saw it emptied of around £2m-worth of the online currency, bitcoin, appear to be genuine, with buyers giving the vendors high satisfaction ratings.

Chat on online forums suggests a growing community of people have bought the vouchers. One potential buyer queries whether they could be found out if they used the vouchers. "I'm sure Tesco would notice if someone is ordering all their food with only 50-100 pound vouchers," they write. "I tried it, I bought £100 from Revivalry (vendor on SR) around Christmas time, it worked in store not sure about online never bought online vouchers," another replies.

Tesco insists none of its customers will lose out as a result of the fraud. A spokeswoman said: "We have referred the matter to the police and will assist them with their inquiries."

The fraudulent sale of Tesco Clubcard vouchers has been known about for several months.

Tim Keanini, chief technology officer at Lancope, an IT security firm, said such hacks were becoming common. "These events are about as hard to predict as the sun rising tomorrow morning," he said. "The problem is not that cybercriminals break into these networks; the problem is that they can go undetected while they figure things out and ultimately exfiltrate the files without being seen."

But the fact that the vouchers were the most popular item sold on Silk Road 2 suggests that the dark net is moving into the mainstream.

The original Silk Road site was closed down by US law enforcement agencies. Similar sites with names such as Sheep and Pandora sprang up, raising questions about whether the internet can be effectively policed.

The Silk Road 2 screengrabs show that, before it was taken down, vendors were selling items such as a gram of uncut Peruvian cocaine for $100, while a gram of ketamine was selling for $37.

Gaining access to the dark net – or Tor Network, as it is officially known – is not easy. A specialist browser that protects users' anonymity is needed, and web addresses are complex codes that cannot be found using conventional search engines.

"The amount of money going through these websites is unbelievable," said Gareth Balmer of the treatment charity Addaction, who has been studying the online drugs trade. He suggested the internet would transform drug dealing as it had transformed the distribution of film and music. "Ten years ago, downloading was a niche market. But now it's much easier and more straightforward. It is possible that we are now on the verge of that happening with the dark net markets."