The Peter Moores Foundation - set up by the knighted British football gaming king, music-lover and philanthropist to attract new audiences to artforms like opera, classical music and the fine arts - put up the $27m budget, which would have been almost impossible to raise on the open market. But with the Branagh brand name attached, this is by no means an uncommercial prospect, despite the fact that the stars are singers rather than actors.

'The Magic Flute' will play best in territories, from the US through most of Europe to Japan, where an audience already exists for classic Western opera; if (as seems more than likely) it crosses over to opera newcomers, then will do so within these core countries. Ancillary prospects look promising: the opera DVD market is a small but lucrative one, and this will place it at the widest end of the niche, with longer shelf life than most features. It premiered out of competition at Venice before heading for Toronto.

German-speakers may bridle at the fact that Emanuel Schikaneder's original German libretto has been translated into English and adapted by Branagh's friend and colleague Stephen Fry. But Fry's enjoyably effervescent version - which includes some not overly intrusive lines of informal spoken dialogue, in place of the 'sung-spoken' recitative passages of opera - is well-matched to the music, with the odd comic couplet that reminds one of Gilbert and Sullivan ("I can end the pain I'm feeling/Just by swinging from the ceiling").

There were no English subtitles when the film screened in Venice, but these should probably be added when distributors in Anglophone territories eventually come on board: the singers vary in intelligibility, with the female soprano and mezzo-soprano lines being, as always, the most difficult to decipher.

The plot of Mozart's final opera (written in 1791, a few months before the composer died) is rococo even by the flexible standards of the genre. Its core is the series of trials that a noble young prince, Tamino (Joseph Kaiser) and the damsel he loves, Pamina (Amy Carson), must pass through in order to pass from dark ignorance - represented by Pamina's fierce and jealous mother, The Queen Of The Night (Lyubov Petrova) - to the light of understanding, which is identified with the wise ruler Sarastro (Rene Pape).

Branagh and Fry place Sarastro and The Queen Of The Night on opposing sides of the First World War trench warfare; initially a captain in the Queen's service, Tamino is won over to Sarastro's side when he enters his castle in order to rescue Pamina, who is being kept prisoner there. Tamino's comic sidekick, Papageno (Ben Davis), is a lily-livered bird-catcher: in the film version he becomes the soldier who provides and cares for the canaries that were used in the trenches to detect mustard gas.

Rarely has the phrase "theatre of war" been taken so literally. The Queen Of The Night first appears belting her lungs out astride a caterpillar tank, a marching band (complete with violins) is briefly seen playing the music we hear on screen, the three regal attendants of Mozart's opera become sexy nun-nurses and Sarastro's guards are turned into jerky Nutcracker automatons when Papageno sounds his magic chime of bells.

There are only two main sets: the labyrinthine trenches, where soldiers run endlessly round like laboratory rats, and Sarastro's castle, where production designer Tim Harvey picks up the Masonic subplot of the opera and translates it into a fantasy space that is half bare Gothic cathedral, half arcane Da Vinci Code palace.

The film's studio-based hyper-realism is enhanced by musical-style lighting a la Moulin Rouge and by a whole range of CG effects, at their most impressive in the six-minute opening crane shot (an homage to Welles' Touch of Evil?) where we pan down from the blazing sun to a flowery meadow that turns out to be furrowed with trenches, before swooping and dipping through the trenches and out again to a field where battle formations are being drawn up.

Little attempt is made to conceal the artificiality of this CG work: at times (especially at the end, when green meadows replace the battle-scared badlands) we feel like we've strayed into a Pixar movie. Ingmar Bergman's playful1975 version of 'The Magic Flute' is nodded at in one special effect, when a black-and-white photo of Pamina begins to dance: in Bergman's film, it was her picture in Tamino's locket that came to life.

Branagh disproves the maxim that opera singers can't act with his inventive casting, which mixes young but already established singers like Joseph Kaiser with new talents like Amy Carson, whose performance as Tamina is outstanding, challenging the stylised nature of the exercise with its emotional depth.

With his budget, Branagh could easily have used opera divas like Roberto Alagna and Angela Gheorghiou, two leading lights of the opera world who appeared together in Benoit Jacquot's 2001 film version of 'Tosca'. But while the decision to go for expressive range over opera star cachet may annoy a few hard-line opera buffs, it bolsters the film's cinematic impact no end.