Eye witness: This isn't sport. It's a public execution

Ultimate fighting: Punching, jabbing, kicking, strangling - anything goes.

Mark could kill me with a single kick to the throat, but is too polite to mention it. He's a nice bloke: softly spoken, good manners, loves his mum. And by the time you read this she may have seen him spill blood, break bones or get beaten into submission while fighting a man called the Wolf in a cage at the Royal Albert Hall.

Mark could kill me with a single kick to the throat, but is too polite to mention it. He's a nice bloke: softly spoken, good manners, loves his mum. And by the time you read this she may have seen him spill blood, break bones or get beaten into submission while fighting a man called the Wolf in a cage at the Royal Albert Hall.

Flag-waving on the Last Night of the Proms used to be as rowdy as it got at the Albert, until someone agreed to host the Ultimate Fighting Championship, a new sport from America that some compare to human cock-fighting.

The boxing promoter Frank Warren says it is like a public hanging. The British Medical Association wants it banned, because combatants intend to inflict injury, including brain damage. It is the closest thing you can get to bare-knuckle, no-rules fighting, this side of the law.

The Wolf has a speciality strangle called the Guillotine Choke. Mark does not intend to get caught in it, so strips to his fighting shorts to show me how those long legs can land precise blows at blinding speed. His smile may be gentle but his body is rock hard. Frankly, with his clothes off he is terrifying.

Afterwards we sit in the rooftop conservatory of a posh hotel in Kensington, surrounded by huge men from all over the world who have flown in to compete, and Mark tells me his mother is to blame for getting him into this.

She stopped him boxing as a boy because of all the blows to the head. So he took to martial arts instead, and became the Tae Kwon Do world champion. Then someone brought videotape of ultimate fighting bouts home from America.

"Eight men enter, one man leaves," said the flashy, all-action presentation. "A blood sport to the death." Behind the hype, Mark saw an astonishing sport that combined the martial arts he was good at with the skills of boxing, judo, wrestling, kung fu and many other disciplines including the Brazilian vale tudo, "anything goes".

Now the 33-year-old from Gloucester rises at 5.30am to train for six hours a day, every day, in his own gym, called the Pit. His mother flew in from Jamaica to watch last night, or possibly to hide her eyes and reflect on what happened to his career as a civil engineer.

When ultimate fighting was invented in 1993 there were only three rules: no going for the groin or throat, and no gouging of eyes. Now they ban biting, hair-pulling and even "unsportsmanlike conduct". Kicking an opponent in the head when he's lying down is apparently OK, if he can defend himself. You win if the other guy gives up (he can tap the floor if unable to speak) or the ref stops the fight.

Thank God for rule 29, is what I say: "No timidity, including avoiding contact with an opponent." That's me out then. Running away is forbidden; and the cage is locked during the fight. Otherwise, anything goes. Kick, punch, pull, twist, jab, slam, wrestle or strangle. The best arm or knee locks make the victim black out in seconds; the side choke will kill in 20 if the ref doesn't intervene.

Despite all this, ultimate fighters say they are safer than boxers. The worst injury suffered so far has been a punctured lung. "If you can't defend yourself the ref steps in," says Mark. "It is honourable to tap out – but I have seen boxers go to their corner, get patched up and called names, then pushed out there to take another beating."

The reason for this civilisation (some say taming) is the desire for respectability, and cash. Promoters used to boast of being banned in 49 US states, and everyone agreed the spectacle was akin to watching a car crash, but broadcasters and advertisers thought it all too hardcore. Last year the original organisers sold their tournament to a company from Nevada and ultimate fighting began to acquire the trappings of a proper modern sport: rules, accreditation and a slot on Sky Box Office.

Elvis Sinosic is a big Australian with close-cropped hair dyed green and yellow. His T-shirt proclaims him as the "king of rock'n'rumble", and I'm not about to argue. Elvis – his real name – is an IT consultant but he hopes to go full-time as a fighter. "When I first started promoters just put you up for the night. Now they are flying us from all over and paying appearance money – though you still have to pay for training partners, masseuses, gym time and equipment."

Ultimate fighters train as hard as any athletes but are used to being treated as freaks by outsiders. What counts is the competition. "None of us are getting rich," says Elvis. "The only thing you get for all the years of effort, and for getting into that cage, is respect."