UK law states that for Cars and Light Commercial Vehicles the minimum groove depth of the tread pattern shall be of a depth of at least 1.6 mm throughout a continuous band comprising the central three quarters of the breadth of tread, around the entire outer circumference of the tyre.

However at 1.6mm tread depth it takes 13.6 meters  longer to stop in wet conditions than with new tyres Therefore the Tyre Industry Council strongly recommends changing tyres at 3mm and below.