Fentanyl is up to 100 times stronger than heroin and carfentanyl is 10,000 times stronger again.
Jake Levene, 22, Lee Childs, 45, and Mandy Lowther, 21, sent fentanyl and carfentanyl to customers in the UK, France, Spain, Germany, Holland, Norway, US, Canada, Australia, Argentina and Singapore.
They created a website called UKBargins on the Darkweb and made £163,474 (US$217,000) between December 2016 and April 2017.
One of The Gang Nearly Died While Making It
Six of their customers died from taking fentanyl and Lowther almost died while producing the drugs.
He was taken the intensive care unit of Leeds General Hospital in February 2017 and fell into a coma.
He suffered a brain injury which was linked to exposure to fentanyl and carfentanyl.
Despite his near-death experience he continued to produce and supply the deadly drugs.
Britain's National Crime Agency (NCA) said the gang sold to 172 people in the UK and 271 elsewhere.
The gang were based in an industrial unit in Morley, near Leeds, and they bulked up the drugs with other chemicals.
Levene and Lowther controlled the unit while Childs packaged and posted the drugs to customers.
Childs was captured on cameras at various post offices sending hundreds of drug packages through the mail.
Levene and Childs admitted exporting and supplying class A drugs and Lowther admitted the same charges at Leeds Crown Court on Tuesday, May 29.
In April 2017 the NCA and West Yorkshire Police raided the unit and found fentanyl as well as a vacuum sealer, funnels, digital scales, and areas for mixing, blending and heating.
Millions Of Lethal Doses Discovered
They also found 677 grams of pure carfentanyl — the equivalent of millions of lethal doses.
The gang's website — which used the AlphaBay Darkweb market — included a message.
"I will not give any information about fentanyl or its analogues as the customer should already of researched these chemicals before even contemplating using them as they are extremely dangerous and lethal in the wrong hands," said the clumsy disclamer.
In Levene's car they found a "to do" list which included the notes "pay electricity bill" and "find new CF (carfentanyl) supplier."
"This operation has resulted in the closure of an organised crime group sending horrifically dangerous drugs across the world, and the jailing of the men behind it. They knew exactly how lethal the drugs were but continued to sell them," said Greg McKenna, regional head of investigations at the NCA.
There have been more than 120 deaths relating to fentanyl or carfentanyl in the UK since December 2016.
"We have taken out a main supplier but the threat from synthetic opioids remains and we will continue to respond to this UK-wide threat with our law enforcement partners," said Mr. McKenna.
All three will be sentenced in September.