Worn or damaged safety belts can be unsafe. They will probably be identified at your next warrant of fitness (WoF) or certificate of fitness (CoF) inspection. But for your own safety, get worn or damaged belts fixed as soon as you find any faults.
Fraying and cuts to the belt material (also called the 'webbing'). Even a small tear can decrease strength by up to 60%. In a crash the belt could snap.
Fading of the belt. Prolonged exposure to sunlight reduces a belt’s strength and as a result it could snap in a crash.
Damaged buckles. In a crash, a damaged buckle could release the belt.
The belt won’t retract. The retractor makes your belt fit snugly. In a crash, a damaged retractor may fail to stop you from being thrown forward into the steering wheel, dashboard or windscreen.
If you have an older vehicle with worn or damaged safety belts you may have to replace them with webbing clamp safety belts in order to pass your WoF or CoF inspection.