Considering a case where a cyclist and motorcyclist collided (Smith v Finch 2009), Mr Justice Griffith Williams ruled that the cyclist could have been found partly liable if wearing a helmet would have prevented or reduced his or her injuries.

In this particular case, it was accepted that a helmet would not have protected the cyclist, Robert Smith, because of the speed at which he hit the ground.

But Richard Brooks from law firm Withy King told BikeRadar that this ruling means that if you are injured and a cycle helmet could have reduced your injuries, you may not be able to recover full compensation.

Cyclists who "expose themselves to a greater degree of injury" by not wearing a helmet can now be found to be negligent, even though it is not a legal requirement in the UK to wear head protection when cycling. However, for this to happen it would have to be proved – using medical and other evidence – that a helmet would have prevented all of their injuries or made them a good deal less severe.

In this case, Mr Smith was involved in a collision with a Yahama 600cc motorcycle ridden by Michael Finch in June 2005. He wasn't wearing a helmet and suffered serious head injuries, leaving him with no recollection of the accident .

He claimed damages from the motorcyclist for personal injuries, and the biker then brought a counterclaim, claiming that Mr Smith was liable for his own injuries because he had a helmet but had not worn it. The court found the motorcyclist primarily liable, saying that on the balance of probabilities Mr Finch had been speeding and riding too close to Mr Smith as he tried to overtake.

The judge then considered whether Mr Smith had contributed to his own injuries by failing to wear a helmet. He heard that Mr Smith's injuries were caused both by him hitting the back of his head on the ground and also from rapid rotation of the skull as he came off his bike, causing blood vessels to rupture.

Helmet expert Dr Bryan Chinn examined Mr Smith's helmet, which was about 20 years old, and told the court that neither that model nor a more modern one would have prevented Mr Smith's injuries because he hit the ground in excess of 12mph. He said the scalloped shape of most modern helmets would not have prevented Mr Smith's injuries, given the location of the impact on the back of his head.

The judge said that, in the absence of expert medical evidence – which he called a "fundamental evidential omission", the court accepted Dr Chinn's evidence and the motorcyclist was fully liable.