"That the best you can do?" he taunts, even as he struggles to remain upright.

"Bring it on!"

The star of Men In Black, Independence Day and Wild Wild West keeps swinging and keeps taking punches until the bell rings and he collapses exhausted into a corner. . .

It was another day, another painful lesson as Smith endured the toughest training programme Hollywood has ever seen to add 30 pounds of solid muscle to his lithe 6ft 2in 185lb frame. He suffered this regime to transform himself into a world champion boxer for his GBP 70million blockbuster movie, Ali, which has its world premiere in London tomorrow night.

"To understand the sacrifices that Muhammad Ali went through to be the greatest boxer ever Will decided he had to go through it all as a fighter - the pain, sweat and tears, the dieting, abstaining from sex and the intense discipline, " says Smith's boxing and fitness trainer Darrell Foster. "He really became Muhammad Ali."

"Will vowed to have no sex for the year he was training for Ali, " reveals Foster. "Sex saps a fighter's energy and Ali always abstained through fight training. Will wanted to really live the role, experience it all.

"I wasn't just training Will for a movie. I was preparing him for a world title bout. My objective was to teach him how to fight, period. I wanted to instill in him a boxer's instinct." But Smith's real fighting spirit shone through when he took beating after beating in the ring.

"Will never once backed off, " says Foster. "He never gave in. Even when he was hurting, he kept on going. There were times he went home and cried. He didn't know if he could take all the beating, month after month. But he kept coming back. He never let it show.

Ninety per cent of boxing is heart - and Will Smith has that and more."

The biographical movie follows the pugilistic legend from his days as lightfooted Cassius Clay, winning the world heavyweight championship from Sonny Liston in 1964, through his conversion to Islam and battle with the US government over his refusal to go to Vietnam, to the day he regained the championship from George Foreman in 1974.

Smith faced the daunting task of not merely looking like Ali but moving and fighting like a genuine world champion.

"Director Michael Mann wanted a fight movie, not a stunt movie, " explains Foster, an intimidatingly muscular 43year-old, who trained former world champion boxer Sugar Ray Leonard for 15 years and has since trained Antonio Banderas, Woody Harrelson and Ving Rhames for boxing movies.

"He'd have fired me if I had tried something theatrical and unrealistic like the climactic scene in Rocky. There are no funny camera angles or fake blows in this film. Every blow hit, so Mann wanted Will toughened up to take all that punishment. The first few weeks I roughed him up. I was trying to hurt him, to see if he had the heart, if he would keep coming back. We had to know early on if he could take the punches.

"He never let on that he was in pain, though I know he told friends he was suffering. But he wasn't worried about getting hurt, so why should I worry about it? We didn't baby him because that would give him a false sense of security. Even during filming I'd tell his opponents: 'This is your first movie, your big break. You'd better go for it'.

Their aggression gave us the realism we needed."

Smith had to learn from scratch how to box. "Will thought he knew how to fight because he had been in some street fights, " laughs Foster. "But really, he couldn't fight at all."

HE COULD take a punch, however. "I knew I had a chin, " the 33-year-old former rap star explains. "That's 90 per cent of boxing - not being scared to be hit." But after months of sparring with Foster, the trainer began putting Smith in the ring with former world boxing champions including British-born Michael Bentt, the WBO heavyweight titleholder in 1994 who also plays boxing champ Sonny Liston in the movie - and gave Smith a wake-up call he will never forget.

"One day I got a little big for my britches, " recalls Smith. "And Michael returned the favour." The blow almost knocked the actor out cold.

"I felt an electric shock from the base of my neck down to the back of my elbows. That was a rite of passage - just standing up afterward made me feel like a beast."

But Smith kept coming back for more. "He endured months of pain in training and filming, " says Foster. "He broke his right thumb, his ribs were constantly pummelled, he'd have bruised lips and arms. His body was sore all the time.

"He was reduced to tears but he'd never let me see it. When he could see that he could fight, and he was morphing into Ali, his emotions would sometimes overwhelm him with the enormity of the role and the responsibility to a legend.

"Ali himself came to the gym one day to watch Will train. He shook his head as he watched Will take all these punches and keep dishing it out, dancing around the ring and flicking off jabs, mouthing off, and he said: 'That boy's crazy.' " Smith found Ali's character through his intense fight training. "Darrell taught me how being a fighter shaped Ali's psychological profile, " Smith explains. "The only road was to learn how to fight, to feel the fear of trying to bash an opponent's head in, to experience what it feels like before a big fight. I needed to understand what it was like to be attacked in the ring, because that helped me to know what it felt like to be attacked by the US government."

Smith began his transformation into Ali by changing his diet. "We originally put Will on a high-protein, high-carbohydrate diet to rapidly increase his size. When we came closer to shooting, he switched to a highprotein, low-carb diet to look leaner.

"But we filmed out of sequence and that caused problems. One week we'd shoot a 1964 scene where Will had to be lean and muscular. The next week we'd film a scene from 1974 when Ali was still strong but slightly bloated - so Will went on a high sodium diet: lots of sushi with soy sauce and salted foods.

"Even last Christmas when Will went to Aspen, instead of taking time off he invited me up there for high-altitude training. I had him in combat boots running and boxing knee-deep in the snow, 8,000 feet up."

But turning Smith into a convincing boxer was tougher. "I taught him how to take a punch, not fake a punch, " says Foster, who broke his own right hand during training and fought on with his hand in a fist-shaped cast.

"We ran an intensive fitness programme to get him in fighting shape - he ran five miles each morning, spent three hours learning footwork, balance and ring generalship. He learnt how to throw a punch and improved with a speed bag and a heavy bag. He increased his agility and endurance by jumping rope, then ended the afternoon lifting weights.

"Sometimes I'd tie his hands behind his back so that he had to get out of trouble just using his feet."

FORMER boxing champion Michael Bentt, aged 36 - whose career ended in a disastrous 1994 loss in a London ring, plunging him into a coma - says: "Will captured Ali's boxing style beautifully. The way he flicked his jab like a snake, his ring generalship, his fluid and graceful movements. Even the Ali shuffle. Will morphed into Ali.

"He hurt me too. He threw a right hand that buzzed me and when I watched the tape later I could see my knees buckle."

Actor David Haines, aged 31, who plays the champ's younger brother Rody Ali, often sparred with Smith and says: "His footwork eventually became so sweet he could send me one way and go another, leaving me frozen. It was pure Ali."

Smith's skills lent added realism in the ring. "I'd tell him: 'The camera's over here, fight over there for 30 seconds' and I'd let him fight how he wanted. His training and instincts took over. By the end of his training Will could have turned pro - and he would have won fights, " assures Foster, who is now training Smith to lose weight while staying fit for his coming role in Men In Black 2, while also training Smith's wife, actress Jada Pinkett, for a fighting role in the two sequels to The Matrix.

"Remember, he had been trained by the best, only sparred with world champions and dedicated a year of his life to the ring. His hand speed was phenomenal, and his right hand could be devastating. He might never be a world champion, but on screen even the real Ali will admit he's certainly the greatest."

Smith grins: "Now, I know that anyone I walk by, I can pretty much beat their arse."