A terrifying video has emerged of a motorcyclist crashing during a race after he was hit by flying debris and knocked unconscious.

The footage, shot by another racer on June 3 at Snetterton Circuit in Norfolk, UK, captures the moment the bellypan from a bike ahead of him hit Joshua Boyd on the head knocking him out and leading him to crash.

After the impact with the bellypan, Boyd remained on his seat for several metres before he eventually fell to the floor.

"The bellypan made impact with my head at speeds easily exceeding 140mph," Boyd wrote online.

"Luckily, I have no memory of the incident, only that I had left pitlane and woke up in hospital with blurred vision coupled with seeing different colours in each eye."

"I have been extremely fortunate to come away from the crash without serious injury," he added.

A crowdfunding page was set up to help Boyd repair his bike.