Thousands of donkey skins have been listed for trade on ecommerce sites and social media.
The trade is estimated to involve the trafficking and slaughter of 4.8 million donkeys annually, according to The Donkey Sanctuary (TDS) charity.
The skins are used in the production of ejiao, a traditional Chinese remedy.
In a new report into the illicit trade, TDS revealed that thousands of donkey skins are for sale online, including on social media giants such as Facebook and Twitter.
Dr Ewan Macdonald, postdoctoral research fellow at Said Business School and co-author of the report, said almost 20% of donkey skin traders operating online also sell other wildlife products, including elephant ivory, pangolin scales and big cat parts.
TDS has called for online retailers and social media platforms to ban the sale of donkey skins and to take down existing listings.
Marianne Steele, acting chief executive of TDS, said: “There is no doubt that the global ejiao trade is having a devastating impact on the welfare of donkeys around the world, who suffer at every point from source to slaughter.
“This new research demonstrates just how vast the online donkey skin trade has become, and how entrenched it is with criminal activity and other illegal wildlife trafficking.
“By cracking down on the sale of donkey skins on their platforms, ecommerce and social media sites will not only prevent considerable cruelty to donkeys, but also help eliminate the other criminal activity that is taking place alongside it and support thousands of families who rely on their donkeys to survive.”
Facebook and Twitter have been contacted for comment.