Experts have warned that more than a million motorists could be at risk of serious accidents and prosecution after buying substandard used tyres to save money.
New figures show 1.5 million drivers have purchased tyres illegal to use on the road in the past five years.
Police have responded by waging a campaign against drivers caught with defective tyres. The number of drivers pulled over for using illegal tyres has soared 9% since 2009.
Officers can impose roadside penalties of up to £100 and three penalty points for each substandard tyre.
Despite the law saying used tyres must be stamped "part-worn" to prove that quality control and safety checks have been undertaken, only two out of five people who bought second-hand tyres found them to be appropriately marked.
Inadequate tread can cause skids and significantly increase a vehicle's stopping distance and fuel consumption, according to a report commissioned by insurers LV.
Of the 23 million used tyres sold since 2009, a quarter did not meet safety requirements. The number of trading standards investigations into part-worn tyre dealers has almost doubled in the past five years.
John O'Roarke, managing director of LV car insurance, said: "In the current economic climate, motorists are understandably looking to cut costs wherever they can and buying quality used tyres is one way to do this.
"Unfortunately some second-hand tyres sold do not meet the legal safety requirements and are not fit for the road, putting motorists at risk of a criminal conviction or worse."