Clothing retailers have been told to take immediate action to ensure items they advertise as containing fake fur are not made from real fur.
The UK's advertising watchdog has given them a deadline of 11 February, after which they may face sanctions.
Last week the Advertising Standards Authority found online retailer Boohoo had sold a pompom jumper which used real fur, most likely rabbit fur.
It's part of a more widespread problem of real fur masquerading as fake fur.
A pompom headband sold by Zacharia Jewellers, a firm trading on Amazon, was also found to have broken the rules. Last year a BBC investigation found TK Maxx and other Amazon retailers had sold items labelled faux fur but using real fur.
The items were spotted by animal welfare charity the Humane Society International as part of an ongoing investigation into the trend.
Real or fake?
Real fur, while traditionally considered a luxury material, can sometimes be cheaper than artificial fur. As a result some manufacturers have used fox, racoon or rabbit fur on items without accurately labelling them.
The findings against Boohoo and Zacharia prompted the Committees of Advertising Practice (CAP), which draws up rules on advertising practice, to issue an enforcement notice.
It requires retailers to ensure they are not misleading consumers and provides some advice on how to go about examining products and supply chains more closely.
Laboratory testing was the most reliable method but the CAP said customers and retailers could also check items themselves using relatively straightforward methods. These include looking at the base material to see whether it is a natural leather or a woven fabric, and burning some of the fur to see whether it singes or melts.
The CAP said it didn't take a view on the ethics of animal fur as a consumer product, which can be legally bought and sold in the UK. But it said making misleading advertising claims that it was "faux fur" when it wasn't did breach the rules.
Guy Parker, chief executive of the Advertising Standards Authority (ASA) said consumers who thought they were buying faux fur "in good conscience" could find it "deeply upsetting" to find they had actually purchased animal fur.
The Humane Society (HSI), said it was delighted its complaint had been upheld but there was still "a shocking amount of fake faux fur" for sale in the UK.
Claire Bass, executive director of HSI, said consumers had "the right to be confident that when they buy faux fur they are not being duped into buying the exact animal cruelty they are trying to avoid."
Companies which don't comply could end up being referred to Trading Standards for potential prosecution.