Andrew Bomford from the BBC's PM programme reports.

The message used to be very simple. Government information campaigns focused on tragedy, and there was a clear, indisputable conclusion: "Wear a helmet - it could save your life", the adverts said.

The evidence seemed clear - experts claimed cycle helmets would reduce fatalities by up to 90%. Road safety campaigners pushed them vigorously, and governments responded.

Australia was the first country to make them compulsory, then New Zealand, parts of America and Canada followed.

Cycle helmets are not compulsory in Britain

The logic is inescapable - try riding a bike through the busy streets of London, and it would seem almost perverse not to wear a helmet. 40% of cyclists do.

London has the highest helmet wearing rate in Britain, so you would think that the numbers of serious head injuries would be falling. But you would be wrong.

In fact in the same period that the numbers of people wearing helmets has been increasing, so too have the numbers of serious head injuries.

The equation is even stronger when you consider what experts call the severity ratio - that is the numbers of head injuries per cyclist on the road, which is considered to be a more accurate measure.

False sense of security

Mayer Hillman, of the Policy Studies Institute in London, is a keen cyclist, and the author of numerous reports on cycle helmets - and he refuses to wear them.

"The real problem is that the road safety lobby has been promoting helmet wearing for twenty or thirty years, " he says, "It is very difficult for them in light of this evidence to reappraise the conclusions they reached years ago and do a U turn so to speak."

Dr Hillman believes in "risk compensation", that helmeted cyclists are lulled into a false sense of security and therefore take greater risks than their more vulnerable counterparts.

One thing is clear - in every place that helmets have been made compulsory, the numbers of cyclists have fallen.

Only 14% of Britian's children wear helmets

Another independent expert, John Franklin, has analysed the data in Australia and found that while head injuries have fallen by eleven percent - there are 15 percent fewer cyclists on the roads, suggesting the risk has actually increased.

It's an argument accepted by Vivienne Nathanson, head of Health Policy at the British Medical Association.

The evidence from various countries that have introduced mandatory helmet laws is that people give up cycling

The BMA's Vivienne Nathanson

She says: "The evidence from various countries that have introduced mandatory helmet laws is that people give up cycling - both children and adults. Clearly we are very concerned about people exercising - to lose that exercise would be terrible. People don't make it up somewhere else."

At a focus group for young teenagers, it was easy to see why only 14 percent of children wear cycle helmets.

One youngster said: "They're a fashion crisis!" The consensus was that they were poorly designed, unfashionable, and uncool. Until their friends wore them, most said they would not.

But the group became silent when they met 22-year-old Darren Sharp, who eight years ago suffered serious and lasting head injuries after hitting a car while out on his bike.

Darren has problems walking and breathing, and his speech is slow and halting. "I was cycling along a road I usually go fast down, and I went to turn a corner and I just hit a car head on," he said... "I'd like to go back to 10 June 1992," he added, "And make sure I've got a helmet on."

Darren helps out at the Bicycle Helmet Initiative in Reading, Berkshire.

t's an organisation that claims great success in persuading school children to wear bike helmets.

Any reduction in damage to the brain is a plus

Angie Lee, Reading Bicycle Helmet Initiative

"Any protection in any crash, and any reduction in damage to the brain is a plus," says its Executive Director Angie Lee.

"That can make the difference between someone living a life that is full and active to someone who has permanent disability".

Angie is a nurse who has seen plenty of head injury cases at the local hospital.

Look at any stage of the recent Tour De France and the professionals almost all wear them.

And that is despite the fact that helmets can only protect the head in impacts of up to 12 miles an hour. Its evidence perhaps that the logic of the old adage "better safe than sorry" is hard to beat.