Will Smith yesterday brought one of Hollywood's more protracted casting sagas to a finish when he finally signed to portray Muhammad Ali in Columbia's big-budget biopic of the boxing icon.

The film will follow Ali from his fledgling days as humble Cassius Clay, through his controversial conversion to Islam, to his opposition to the Vietnam war and his career-defining Rumble in the Jungle against the hotly tipped George Foreman. Michael Mann will direct and Variety reports that Smith has already begun training for the role. In line with his A-list status, the Men in Black star will receive an advance of $20m for the role against 20% of the gross.

Still a few months off filming, the Ali biopic already has a well-thumbed look about it. The project was originally discussed 18 months ago as a collaboration between Smith and Barry Sonnenfeld, the star-director partnership behind Men in Black. But Sonnenfeld bailed out last summer, prompted - some say - by the disastrous reception to the last Smith-Sonnenfeld picture, Wild Wild West.

Earlier this year, the film was the subject of a heated tug of war between Spike Lee and Michael Mann. Columbia execs eventually chose Mann, regarding him as more likely to make a populist, successful picture than the maverick and more pointedly political Lee.

The one constant throughout all this wrangling was Smith, who was always Ali's own preferred choice for the role. But the actor's dithers over signing were rumoured to be causing anxiety to the studio, who still aim to have the film completed in time for a release in summer 2001.

Insiders have speculated that Smith hesitated because he has recently been torn between the Ali film and a Men in Black sequel alongside Sonnenfeld. A spokesperson for the actor denied that this was the case.