A joint five-year investigation by TyreSafe and Trading Standards revealed that only 13 of 152 part worn tyre providers were selling roadworthy tyres to motorist customers.
Out of 152 part worn retailers, 139 were found to be selling tyres which are deemed illegal and unsafe.
TyreSafe suggests their investigation highlights how retailers in this sector are not responsible enough, or do not possess the necessary skills, to properly serve UK motorists.
Faults detected by the investigation team during test purchases included: tyres fitted with water in them, wrong sizes of tyre being provided, as well as examples of tyres with nails or other objects embedded in them.
Following this investigation, a part worn retailer in Hemel Hempstead was prosecuted by Hertfordshire Trading Standards and ordered to pay fines and costs over £7,000.
Magistrates, in giving their verdict, stated they were "shocked" at the case and the wider implications of unsafe tyres being sold. A custodial sentence was considered as a potential penalty, with the retailer being warned that any further offences would likely result in prison time.
Chairman of TyreSafe, Stuart Jackson, said: “TyreSafe applauds the magistrates’ comments and penalties in this latest conviction, but it must be acknowledged that the retail of dangerous and defective tyres by part worn dealers is unacceptably commonplace nationwide.
“While the shocking findings of joint investigations may reveal some part worn dealers are compliant, even if it is fewer than one in 10, motorists have a 90% chance of visiting an outlet selling illegal tyres. As far as TyreSafe is aware, there is no other retail sector with such an atrocious track record.
“Let’s remember these dealers are selling safety critical components - when tyres are driven in an unroadworthy condition, a vehicle’s braking and steering are compromised, and road users are put at significant risk of a catastrophic tyre failure.