They suggest that manufacturers should offer a wider range of styles and sizes as this is currently limited and some people make do with ill-fitting headgear.

The researchers, from the Harborview Injury Prevention and Research Center in Seattle, questioned more than 1,700 cyclists of all ages who had worn a helmet when they were involved in a crash.

They found that overall an ill-fitting helmet doubled the risk of a head injury when compared to one with a good fit, although they provided a greater level of protection than no helmet at all.

Boys were most likely to have poorly fitting helmets, and almost 60% of children under 10 had a difference of 1.25cm or more between the size of their head and the fit of the helmet.

Risk tripled

The researchers came across a significant number of accounts of helmets tilting backwards at the time of the crash or falling off altogether - 13% and 4% respectively.

While a backwards tilt doubled the risk of a head injury, those that came off tripled the risk.

The researchers also compared head sizes and helmet measurements, and found that almost half of children who had sustained head injuries had helmets that were significantly wider than their heads - two centimetres or more.

They said that helmets were important to help prevent cycling injuries, but ill-fitting helmets could not deliver maximum benefits.

This was because if a helmet is too large there is more room for the brain to accelerate before it comes into contact with the protective lining.

Training for sales staff

Publishing their findings in the journal Injury Prevention, the researchers said: "Poor fit of helmets may be associated with an increased risk of head and brain injury.

"It is clear from this work that helmets do not fit some children well.

"Helmets may need to be redesigned, particularly for the younger age group, to fit better - in particular by decreasing width."