Bald and worn tyres could endanger hundreds of thousands of drivers as winter conditions set in.
Figures based on research carried out by Micheldever/Protyre supported by the Vehicle & Operator Services Agency showed that half of cars undergoing an MOT test required at least one replacement tyre because of illegal tread depth.
The checks, conducted over the month of September, also revealed the number of cars on the cusp of having illegal tyres amounted to a third of the total.
The legally required 2mm tread depth for a successful MoT test was seen on only 10% of vehicles.
Similar results were found in 2009 with 2.3 million failures due to tyres that were not roadworthy.
The punishment for driving on illegal tyres is a £2,500 fine and three points on the driver's licence, a penalty that could increase the driver'scar insurance.
Micheldever/Protyre have launched a campaign calling on drivers to wise up to the dangers of poor standard tyres.
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