Experts have described new evidence of a trade in dangerous and counterfeit crash helmets as "shocking" and "a tragedy waiting to happen".
An investigation for ITV News found evidence of an illegal market in poor quality imports being sold cheaply online, which the authorities seem powerless to halt.
The new findings have led to warnings from police and trading standards, along with calls for more action to stop the unlawful sales.
Adverts in online market places and social media offer helmets for as little as a tenth of the usual price with reassurances that they have full safety certification.
Many are counterfeits using the logos of reputable brands, while others are simply low quality imports.
Mark Mayo of the British Standards Institution - the place that literally sets the helmet standard - explained to ITV News how to spot a fake:
Look out for the helmet's E-number - this shows the product has been safety tested in Europe
If there isn't an E-number, don't buy the helmet
Use a reputable supplier
Look at the helmet yourself, in person, and check it has the right markings
Spend an appropriate amount of cash - if it looks cheap, don't buy it
Tests conducted for ITV News by BSI show how little protection counterfeit helmets provide.
A counterfeit AGV helmet broke apart and split during a test designed to simulate a crash at just 30 mph.
Mark Mayo of BSI said: "The helmet split from front to back, the rest of the head is then exposed to further impacts which will just cause even greater injury.
"The results are quite shocking, to think there are people out there with those helmets on thinking they are getting protection is very worrying".
A fake Arai helmet also split on impact, with parts detaching.
"Fatalities will ultimately result from people wearing that helmet and expecting to be protected," said Mark Mayo, "its a potential tragedy waiting to happen".
While the two fake helmets tested failed all four safety tests, the real one passed every time.
Dangerous helmets sold with fake safety markings have been discovered in use.
Motorbike officer John Wilton of Devon and Cornwall Police was alarmed to find a biker wearing one with a novelty design and invalid safety markings.
He described it as "an abysmal helmet, very cheap - very flimsy".
The officer told ITV News buyers are being misled: "I can appreciate how the wool is pulled over your eyes.
"They go largely off the description they read on the internet and believe they are getting helmets that are legal."
Trading Standards told ITV News they believe this is an emerging problem which is likely to grow but that many of the online sellers are beyond the reach of UK authorities.
Responding to the findings, Selina Lavender of the Motorcycle Action Group said: "Really we should be telling people to watch out - they are not just faking a fashion item here."
"Generally it's people with less money to spend and those new to biking that are going to be buying the cheaper and inferior products...it's affecting those who perhaps don't have the knowledge, so the story needs to be out there”.
Example of a fake:
The group wants new warnings added to the international alert system used by Trading Standards.
"A crash helmet needs to serve a purpose. It’s considered a safety item and really you need to be protected".
Ebay, where some of the adverts were featured, told ITV News "counterfeits are not welcome on eBay and we’re committed to combating their sale.
"Using a combination of technology, enforcement and strong relationships with brand owners, eBay has consistently been an industry leader in working to stop the sale of counterfeit goods, which is a global issue – both online and offline.
"In the rare case a buyer believes that he or she has purchased a counterfeit item, eBay’s Money Back Guarantee applies to virtually all transactions and will cover them accordingly."