The project examined activity on Weiku for six months. It found that while searching for “fentanyl” on the website returned zero results, the website offered related searches such as “Fentanyl,” or “Fentanyl patches”.

Some of those advertising under these search categories had their accounts deleted, while others still sold the substances.

Those who analysed the website were able to find sellers willing to ship fentanyl and other substances. The drug appeared frequently on the website over the period analysed and, as well as fentanyl, Weiku also hosted people selling bromadol, which is estimated to be five times the strength of fentanyl.

One seller of this drug on the website produced a US certificate for a dog examination in lieu of a business licence.

Sellers on the site also advertised pentobarbital, a short-acting barbiturate that can cause death when used in high doses.

It is used in euthanasia programmes in the Netherlands and executions in US states such as Texas. One seller promoted the product as “peaceful death”.

When approached, one seller said: “Our fentanyl powder is very pure ... We can send to you safely in the UK. Delivery will be done through UPS or FedEx.” The seller offered 50 grams for US$700.

Weiku is based in the eastern city of Hangzhou. The company said that it regularly checked for sellers of illegal drugs such as fentanyl, and deleted those accounts.

A representative admitted, however, that searches for fentanyl could return results. “It does happen that sellers and buyer create new ways to make deals on our site, but Weiku is not able to discover this right away.”

The firm said it did weekly inspections. “As an information platform, Weiku needs to make sure all information is legal, but we are not accountable for the transactions,” it said.

The findings come as the head of the UK National Crime Agency (NCA) said that vast majority of the synthetic opioids coming into the UK were from China.

The NCA said that most synthetic drugs, including the synthetic cannabinoid spice, came into Britain from east Asia.

“More than 80 per cent of fentanyl in the UK comes from China ... but to put that into context there are a relatively small number of seizures and UK suppliers. In terms of where it comes from globally, the key source is China,” said Vincent O’Brien, head of operations for drugs and firearm threats at the NCA.

He said the drug was mainly sent through the post and was then sold on the dark web, a part of the internet only accessible through a particular browser.

Another Chinese website, DIYTrade – China Product Directory, offered for sale fentanyl and carfentanyl, which is 100 times more potent than fentanyl.

One seller, when contacted, said they offered 10 grams for US$280 or 100 grams for US$1,100. They said: “Yes, we can ship carfentanyl to London ... how many ... do u need?”

DIYTrade did not respond to requests for a comment.

O’Brien said there was little production of fentanyl and similar drugs in European countries. “China has made progress in banning fentanyl and its analogues but one reason [these drugs] are a concern for us is that there is a new supply route and methodology,” he said. “Fentanyl is supplied by post or fast parcel and because it so potent, a few hundred grams could be used to produce a much higher volume of heroin ... Supply is over the web, particularly the dark web.”

Another Chinese website, Mfrbee.com, had dozens of listings under the search term “opioids”, although links to fentanyl appeared to be blocked. Other opioid analgesic drugs are sold with shipping options.

The website also hosts vendors purporting to sell Xanax, a benzodiazepine, and pregabalin, a medication used to treat epilepsy and generalised anxiety disorder. Both are illegal without a prescription in the UK.

Mfrbee said: “Regarding illegal medicine, we immediately delete those posts after customers report them or after we discover them but some [sellers] can evade this but changing keywords.”

The company said it provided a platform for sellers and buyers and did not conduct the transactions itself. “Our mission is to connect sellers and buyers around the world to do legal business. We reduce the costs of getting information and promote fair and legal business,” it said.

The market for synthetic drugs in China is driven by demand abroad. The authorities control fentanyl, but they did not begin restricting its two most common ingredients until this year, more than a decade after the US.

The law in China has got tighter in recent years, however, with more than 130 synthetic drugs – including at least 10 fentanyl analogues – added to the country’s list of controlled substances since 2015.

Chinese regulators say they struggle with the speed at which chemists are able to produce new variations on fentanyl, circumventing the government ban on analogues.