Half of the used tyres being sold in Brent and Harrow posed a serious safety risk to road users, an investigation has found.
Trading standards officers in the two boroughs bought second-hand tyres from 12 different traders this summer in a test purchase operation.
Of those, all but two were deemed to be illegal by an expert at TyreSafe, a charity promoting tyre safety.
At least six of the tyres examined had defects which the charity said could pose a "serious safety risk" should they ever make it onto the road.
The latest investigation, which was also supported by the National Tyre Distributors Association (NTDA), follows the convictions of eight second-hand tyre dealers across the two boroughs between August and September last year.
Brent and Harrow Trading Standards said it was providing guidance to those dealers caught selling illegal tyres this July and would assess on a case-by-case basis whether prosecutions were required.
'Buying part-worn tyres could cost you your life'
Councillor Michael Pavely, Brent Council 's cabinet member for stronger communities, said: "Part worn tyres can be very cheap and the savings can be seductive, but it's not worth it if it ends up costing you your health or even your life.
"This investigation by Trading Standards highlights once again the importance of checking your vehicle's tyres.
"If you must buy second hand tyres, make sure you go to a reputable trader and check them before you buy to make sure they're road safe."
TyreSafe chairman Stuart Jackson said the latest investigation indicated a "wilful disregard" for motorists' safety shown by second-hand tyre retailers.
"It's shocking and unscrupulous but having been involved in innumerable investigations, some of which are still ongoing, TyreSafe is regrettably well aware this attitude is shared by a significant proportion of part-worn dealers across the country," he added.
He urged motorists to reconsider buying used tyres and said those doing so should steer clear of any not stamped with the words PART WORN as they were unlikely to have been checked.
