What is the difference between MOT defect categories?

Since May 2018, MOT test results in the UK have used a defect classification system to make it clearer how serious each identified fault is. There are four categories:

• Dangerous — an immediate risk to road safety or the environment. The car fails the test and must not be driven at all until repaired.
• Major — could affect safety, put other road users at risk, or impact the environment. The car fails the test and shouldn't be driven until fixed, though it can legally be used until the current MOT certificate expires if it hasn't already.
• Minor — doesn't have a significant effect on safety or the environment, but should still be repaired as soon as reasonably possible. Doesn't cause a fail.
• Advisory — not a current problem, but something that could develop into one and is worth monitoring or fixing before the next test. Doesn't cause a fail.

Only dangerous and major defects result in an MOT failure. Minor and advisory items are recorded for information but don't stop the car from passing. Understanding these categories helps you judge how urgently repairs are needed, rather than treating every fault report the same way.