In 2008, Simon Reynolds took part in a "team building day" organised by his employers, real estate agents Strutt and Parker. The events for the day were kept secret, but employees were given a choice of categories, and Reynolds chose to be "active and energetic". When he arrived at the event's location he was told that there was going to be a cycle race, so he took one of the bikes provided, ignoring a rack of cycle helmets. Some 25m from the finish line he collided with another competitor and fell off, suffering brain damage.

Reynolds sued his employers for compensation, and the high court's full judgment in Reynolds v Strutt and Parker became available for the first time last week. The judge, Oliver-Jones QC, found Strutt and Parker liable for the accident, largely because they had failed to conduct an adequate risk assessment for the cycle race. But their liability was reduced by two-thirds because of Reynolds' "contributory negligence" in two respects: firstly he had cycled in a dangerous manner which brought about the collision, and secondly he wasn't wearing a helmet.

Wearing a helmet while cycling isn't compulsory in the UK, although an attempt was recently made to make helmets mandatory in Northern Ireland, and a new bill is currently before parliament which would make helmets compulsory throughout the UK for children under 14.

But even though it's not presently an offence to ride without a helmet, so there are no criminal penalties, the Reynolds case means that the decision not to wear a helmet can have legal consequences for cyclists who suffer head injuries and sue for compensation. Although Reynolds was injured in a race, the court's decision could apply in the same way to normal road cycling.

The Reynolds case is the first occasion where the courts have reduced a cyclist's compensation in this way, but it seems to be part of a wider trend in the law's attitude towards helmets. The Cyclists' Defence Fund have suggested that deductions for not wearing a helmet are now common in out-of-court settlements of head injury claims brought by cyclists. And when cases have reached the courts in recent years, the courts have held that cycling without a helmet is negligent (at least for the purposes of contributory negligence), largely because it involves ignoring the Highway Code's recommendation that "you should wear a cycle helmet which conforms to current regulations, is the correct size and securely fastened". This conclusion was first expressed by Mr Justice Griffith Williams in the 2009 case of Smith v Finch, and has now been adopted by the High Court in its last three decisions on the issue.

Despite taking the view that riding helmetless is negligent, until the Reynolds case the courts had not reduced any cyclist's compensation, because of problems with the evidence on the effectiveness of cycle helmets.

In 2009 the Department for Transport commissioned a report (requires registration) which reviewed the evidence on cycle helmets and concluded that they "would be expected to be effective" at reducing the risk of head injury in cases not involving other vehicles, or where another vehicle simply glances or tips over a cyclist. But in full collisions with motor vehicles the evidence doesn't establish that helmets are effective, because the applicable standards for cycle helmets don't require testing at impact speeds above about 12mph, which is the impact speed of a fall to the ground from a typical bike seat.

The previous court decisions involved collisions with motor vehicles, at speeds higher than those at which helmets are tested, so the courts didn't have sufficient evidence that a helmet would have made a difference to the head injury. As a result they couldn't make reductions in the compensation awarded to the injured cyclists. Reynolds' case was different because there was no motor vehicle involved, and the judge found that the impact speed was likely to have been within the range at which helmets are tested.

If the Reynolds case goes to appeal it is possible that the outcome may be different. For the moment, though, the case might encourage motor insurance companies who, according to the Cyclists' Defence Fund, may routinely seek deductions when settling head injury claims with cyclists who didn't wear helmets.

The case may also make it more likely that courts will award less compensation in the future to other cyclists who suffer head injuries at low impact speeds while riding without a helmet. But, on its own, the case isn't a reason to change personal decisions about helmets. The court cases on helmets involve life-altering accidents, and show above all that the most important thing is to ride safely, however you choose to do it.

• Jorren Knibbe is a barrister and writes about cycling law at UK Cycle Rules. This article is for information only and is not intended as legal advice.