The legendary filmmaker painstakingly restored and coloured hours of archival footage and paired it with historical veterans' interviews and a special effects soundtrack, to bring the war back to life in an unprecedented way.

This included using lip readers to decipher what soldiers were saying in the century-old film and inserting new matching audio recorded with actors.

The film -- "They Shall Not Grow Old" -- will be unveiled for the first time at the London Film Festival next week, and simultaneously screened in different venues across Britain.

"I was absolutely gobsmacked by the time we'd finished restoring this stuff, I'd had no idea that it could be done so well," Jackson told AFP ahead of the release.