National Tyre Distributors Association and TyreSafe is warning that dealers that sell incorrectly labelled part-worn tyres will be fined, as eight dealers have already been convicted in London.
Six dealers in the Brent and Harrow area of London where fined in August and TyreSafe said two more have now been convicted in North London.
Premier Tyres and Motors and KM Tyres (both from Wembley), and their directors, were found guilty of failing to mark part-worn tyres correctly and selling substandard tyres. One of the tyres was offered for sale with a rusty nail embedded in the tread.
In total, the retailers were ordered to pay £5,238 in fines and costs.
The convictions come as a result of ongoing investigations led by Brent and Harrow Trading Standards with the support of the National Tyre Distributors Association and TyreSafe.
October is tyre safety month, making these convictions particularly significant.
Stuart Jackson, TyreSafe chairman, said: "TyreSafe and its supporters are committed to encouraging motorists to ensure their tyres are safe, so it is extremely disappointing when retailers – who are in a position of trust - are found guilty of supplying dangerous and defective tyres.
"The number of convictions of unscrupulous part worn dealers should cast doubt in the minds of Britain’s motorists on whether the product they are buying in good faith is safe to use at all. TyreSafe research has shown all too often these tyres are not safe and, even when they are, they do not represent good value for money.
“If you are considering buying part worns, TyreSafe encourages you to think again, and at the very least be sure the tyres offered to you are properly marked and in roadworthy condition."
TyreSafe said the vast majority of part worn buyers are those looking to minimise the cost of their motoring, and look at the retail price without considering the dangers nor the lifespan that tyre might offer.
New tyres typically have 8mm of tread, while a part worn need only have 2mm to be legal for sale, just 0.4mm before it reaches the minimum legal depth of 1.6mm.
A previous study by TyreSafe of a random sample of 50 part worn tyres over the course of a month found 98% were sold illegally and 34% were potentially dangerous.