Tyres are considered illegal if they fail to meet certain legal requirements primarily related to safety. The main factors that make tyres illegal include:

• Tread Depth: Tyres with a tread depth below the legal minimum of 1.6mm across the central three-quarters of the tyre and around the entire circumference are illegal. Insufficient tread depth affects grip and safety.
• Damage: Tyres with visible damage such as large cuts (over 25mm or more than 10% of the tyre depth), bulges, or exposed cords are illegal and unsafe.
• Incorrect Type or Size: Tyres that do not meet the vehicle manufacturer's specifications, including wrong size, type, load, or speed rating, can be illegal. Mixing different types, like radial and cross-ply, also leads to illegality.
• Inflation Issues: Tyres that are improperly inflated (either under or over inflated) can suffer damage that makes them illegal.
• Uneven Wear: Excessive uneven wear, such as camber wear, making tyres unsafe.

Driving on illegal tyres can lead to fines, penalty points on your license, prohibition notices, or even vehicle impoundment due to safety concerns. Regular tyre checks for tread depth, damage, and correct inflation are essential to ensure road legality and safety.