UK law requires that your vehicle is fitted with the correct type and size of tyre for the vehicle type you are driving and for the purpose it is being used.
This means fitting the right tyres and for safety ensuring that they are inflated to the manufacturer's recommended pressure.
The legal limit for minimum depth of the tread on your tyres is 1.6 millimetres, across the central ¾ of the tread around the complete circumference of the tyre.
For safety reasons it is recommended that you replace your tyres before the legal limit is reached. Many vehicle manufacturers recommend replacing at 3 millimetres. At 1.6 millimetres in wet weather it takes an extra two car lengths (8 metres) to stop at 50 mph than if your tread was 3 millimetres.
If you are in any doubt as to whether your tyres are road worthy, take your vehicle along to a reputable garage that will be able to advise you.
The law relating to tyres is as follows -
tyres must be suitable for the purpose of their use;
be inflated as to be fit for the purpose for use to which vehicle is being put (your manual will have the recommended inflation for your vehicle);
have no portion of the ply or cord exposed;
have no lump, bulge or tear caused by separation or partial failure of its structure;
have the base of any groove, which showed the original tread pattern clearly visible (this refers to vehicles requiring only 1mm depth of tread i.e. vehicles with a maximum authorised mass exceeding 3500kgs, trailers exceeding a MAM of 3500kgs and motorcycles;
have no cut in excess of 25 mm or 10 percent of the section width of the tyre, whichever is the greater, measured in any direction on the outside of the tyre and deep enough to reach the ply or cord;
the grooves of the tread pattern of the tyre have a depth of at least 1.6mm throughout a continuous band comprising the centre three-quarters of the breadth of the tyre and round the entire outer circumference of the tyre. For the 1mm depth requirement it does have to be around the entire circumference for three-quarters of the breadth but not necessarily the centre three-quarters.