LONDON Mayor Boris Johnson jumped on a bike and zipped down Spring St without a care - or a helmet.

With his mop of fair hair waving in the wind, Johnson didn't even realise he was breaking the law as he straddled the blue council bicycle and took off.

He squeezed past a packed tram and pulled off a sweeping U-turn in the kind of spontaneous display that has earned him global appeal.

London's Mayor showed remorse after he realised he was legally obliged to protect his head.

His only hope, he admitted, was that his mate, Melbourne Lord Mayor Robert Doyle, would understand.

"I didn't see the sign," he said. "I hope we will be all right with the authorities."

Having successfully implemented one of the world's most successful bike schemes in London without helmets, Mr Johnson said yesterday Melbourne should do the same.

He warned that enforcing a head protection policy threatened to kill off the initiative.

"It's not how we would have done it," he said.

"Obviously, it's up to Melbourne to decide what they want to do, but we took a very clear decision that it would be counterproductive . . .

"I wanted to put the village back into London and create an environment where people felt safe cycling and safe cycling in numbers."

He said evidence had suggested that while helmets could prevent some injuries, in the worst accidents in London, "nine times out of 10" a helmet wouldn't make a difference.

In Victoria to be a keynote speaker at the Melbourne Writers Festival, Mr Johnson said Melbourne had all the ingredients to become a cosmopolitan hub.

"I think Australia is the most urbanised country in the world and more and more of us will be living in cities over the next 50 years," he said.

"But people want cities that are clean, green and which have a village feel."