THIS is it – the behind-the-scenes film of the 50-date concert series at the O2 Arena, in London, that sadly never was.

Hastily constructed from hundreds of hours of backstage and rehearsal footage, Kenny Ortega’s documentary is a glowing tribute to the King Of Pop as he prepared for one of the most spectacular live shows of his career.

“Here is a glimpse of the personal gift Michael was preparing for his fans and the audiences of the world,” the sombre opening credits crawl announces against a black screen, which bursts into life with dancers preparing to audition for a coveted role on the tour.

“She’s the one,” says Michael, pointing to a willowy blonde as the show’s director, Ortega, announces the names of the principal dancers to screams and tears of delight.

The rehearsals footage, shot predominantly in June at Staples Center, in Los Angeles, and The Forum in Inglewood, California, opens with a foot-thumping rendition of Bad and segues into Jam, with male dancers flanking the star in perfect time.

The hits come one after another: They Don’t Care About Us with a futuristic military theme, Billy Jean, Beat It, Black And White and a delicate version of Human Nature, which reveals the singer had lost none of the melody in his voice.

For Smooth Criminal, we go behind the scenes of a green-screen video shoot which inserted Jackson into black-and-white movie clips including Rita Hayworth’s glove striptease in Gilda, then a chase involving Humphrey Bogart. A second video shoot for Thriller in 3-D cuts back to the venue, where Michael emerges from a giant black-widow spider that scuttles onto the stage.

A sound-check for The Way You Make Me Feel leads to a difference of opinion with musical director Michael Bearden over subtle changes in key and tempo.

“You gotta let it simmer,” says Michael, trying to let the music build slowly.

All that remains is a triumphant rendition of Man In The Mirror culminating in the epitaph: “Michael Jackson. King Of Pop. Love Lives Forever.”

Screening for a limited two-week engagement, This Is It is an opportunity for the fans to witness arguably one of greatest pop concerts never staged.

A lot of the footage is rough and some of the sound is muffled, but Ortega has cut together scenes with impressive attention to detail, employing split screens when appropriate.

At times, Jackson looks painfully thin and gaunt as he strives to perfect each arrangement, but, for the most part, the film captures a musician at the height of his powers, excited to be reconnecting with fans in a live environment.

Sadness over his death mixes with regret that we never got to enjoy his vision at the O2. If only . . .