Around £800m worth of counterfeit goods pass through online shopping and auction sites in the UK each year, new research has claimed.
Figures compiled by legal services firm DLA Piper and online intellectual property specialists ComSec International suggest that 'e-fencing' has doubled over the past three years.
DLA Piper said that an estimated £5 of every £100 spent at auction sites goes on counterfeit items.
Products most at risk include fashion and cosmetic items, something online auction giant eBay has become familiar with in recent weeks.
The research also warned that auction sites are a second home for 'grey market' trading, in particular the sale of imported consumer electronics, motor equipment and sports goods at highly inflated prices.
"Counterfeit goods dilute the value of brands, undermine the integrity of auction sites and cost the UK economy millions of pounds in lost income," said Simon Levine, joint global head of DLA Piper's technology, media and commercial group.
"But this is not just about protecting brand owners; it is for the consumer's benefit also.
"If you invest your trust and money in a brand you want to know you're getting the real thing, whether that's make-up and cosmetics or domestic appliances and children's toys."
Responsibility for dealing with this problem does not lie solely with the websites, according to DLA Piper, but must be carried by consumers and copyright owners as well.
James Ramm, director of ComSec International, said: "E-fencing poses a significant threat to brands and is a false economy for buyers.
"We use the very latest software to track the infringement of brands online by scanning image and text descriptions, but our data is still only the tip of the iceberg.
"The full extent of counterfeit trading online may never be known and will continue to thrive unless consumers, brands and the sites in question take action against illegal traders."
Latest Tesla news: Tesla stock price tanks amid reports of 'widening probe' by SEC and claims the base Model 3 loses money
SEC 'probe' takes its toll on Tesla as new research suggests that Tesla loses $6,000 on every $35,000 Model 3
10nm Cannon Lake Core i3-8121U CPUs make a rare outing with Intel's NUC mini PC
'Notorious' Australian child hacker thought he had executed 'flawless' hack
The former employee says that Tesla fired him for bringing the accusations to management internally