Image caption Christian the lion lived with his owners in a flat in Chelsea

It is almost exactly 40 years since two young Australians had what has become a famous encounter with a lion in the depths of the African bush.

The huge, powerful animal strode towards the men, and then instead of launching an attack, lolloped up to be hugged, petted and stroked.

The explanation for this extraordinary behaviour is that the men and the lion were old friends.

We realised that we were living with an incredibly complex, intelligent animal John Rendall

Almost incredibly, they had first met in a shop in London. John Rendall and Anthony "Ace" Bourke had come across "Christian" late in 1969, at what was then a zoo in the upmarket department store, Harrods.

"In the centre of it were these two beautiful, beautiful lion cubs," Mr Rendall told the BBC World Service's Witness programme.

He was immediately drawn to the male of the pair.

"He had a nature that was instantly attractive. You could see that he wasn't frightened, he wasn't distressed. He was just above it all. And that is very, very irresistible."

The two Australians, who had only just arrived in London, decided almost immediately that they would buy Christian.

After the Harrods staff had been convinced that they would be the right kind of owners, the friends paid 250 guineas - about £3,500 ($5,736) in today's money - and then walked out of the shop with the lion on a lead.

Remarkable relationship

Mr Rendall said they put their faith in the fact that lions are not solitary beasts. Unlike others in the cat family, they live in groups and have convivial instincts.

Image caption Christian visited the BBC studios in 1969

"If you have a lion by itself in any circumstances it is going to look for friends. And if there are no other lions it will create a friendship with human beings," he said.

And so this threesome began to forge a remarkable relationship, with the cub padding around their London flat, and the furniture shop below where his owners worked.

"We realised that we were living with an incredibly complex, intelligent animal," said Mr Rendall. "He was very observant, always looking."

But what of the dangers to other staff, customers and people passing in the street?

"You think, 'is someone going to come in the door now? Is someone going to bring a child - will they have a dog?'

"It was double-guessing to see that a 'situation' didn't arise. And I'm very proud to say that it didn't happen. You could see that he was a gentle, gentle creature."

And Christian was not living just anywhere. His flat was on the King's Road, the heart of "Swinging London" in the last days of the 60s.

The neighbourhood was home to rock stars and celebrity fashion designers. And as he prowled around the shop, the little lion became something of a celebrity himself.

There were television appearances - when he called by the BBC's studios our security men decided that although no dogs were allowed in, the regulations didn't actually mentions lions - and Christian was welcomed in.

Return to the wild

Image caption Christian soon outgrew his urban environment

But nearly a year on Christian already weighed as much as a man, and he was growing fast. It was definitely time to find him more secure surroundings than his flat in Chelsea.

Eventually the renowned expert in lion behaviour, George Adamson agreed to try to release him into the wild in Kenya.

A year later John Rendall and Ace Bourke decided to go and see how their old friend was getting on. But would he remember them? How would he react?

Just a few weeks earlier another lion in the process of being returned to the wild in the same area had killed a man.

Eventually, in the depths of the bush in the remote Kora region, Christian came over the brow of a hill - now a really large creature with a fine mane.

"He starts walking very very slowly down towards us. The body language was of curiosity - not attack," said Mr Rendall. "Eventually we couldn't resist, and called him - and that's when he took off."

He bounced down the hill and into his friends' arms, letting them hug, play and wrestle with him.

"It was a euphoric moment."

Naive

The whole scene was captured on film, and decades later it surfaced on the internet.

Soon this dramatic revival of an unlikely friendship became a YouTube sensation, moving millions of people around the world.

Media playback is unsupported on your device Media caption Archive: John Rendall and Ace Bourke speak to BBC Breakfast in 2009

In their recently re-published book A Lion Called Christian, John Rendall and Anthony Burke acknowledged that back in 1969 they were naive.

They concede that buying exotic animals only serves to fuel the trafficking of them.

Today Mr Rendall says nobody should try to raise a lion in an urban environment in the way that he did.

The dangers are obvious.

But Mr Rendall was profoundly influenced by his time with Christian.

He became interested in conservation work, and today he is heavily involved with the George Adamson Wildlife Preservation Trust.

As for Christian, his re-introduction to the wild was successful.

He was last heard of crossing the Tana River and heading further north.

And it is very likely that right now there are lions prowling the Kenyan bush who are descendants of Christian the Lion from London.

You can hear Alan Johnston's interview with John Rendall on BBC World Service's Witness via iPlayer or download the podcast.