Amy Carson, who recorded an aria from the 1791 opera with her father on piano in a log cabin in their garden in Taunton, Somerset, and sent it to Branagh's casting director, was hailed by critics as an emerging new soprano.

She was cast as Pamina, straight out of Cambridge University, in what Branagh admits was a risky move.

After opening at Venice's renowned opera house, La Fenice, the first time the venue has been used for a screening, the film is due in the UK next year.

Screen International praised the film, which sets the opera in the trenches of the First World War. It was, the magazine said, truly from the "theatre of war", with the Queen Of The Night "belting her lungs out" astride a tank.

The magazine added that Branagh had disproved the maxim that opera singers cannot act, with Miss Carson "outstanding" as she challenged the "stylised nature of the movie" with an emotionally deep performance. It was pointed out that Branagh, with a £15 million budget, could easily have used a diva, such as Angela Gheorghiou.

The director said yesterday that Miss Carson applied at the last minute. "Sarah Playfair, our casting director, said 'Look, I just have a feeling about this girl.' No one expected very much, because she didn't come with a vast background of experience."

Amy, born in Bristol, comes from a musical family. Her mother, Mary Carson, is a classical and Baroque violinist. Her father, Ian, produced music for BBC Radio 3 in Bristol. At eight Miss Carson became the youngest chorister in the first girls choir at Salisbury Cathedral.

Educated at Bryanston, she said: "My parents knew I could sing from an early age because I was always making a racket. I did Bugsy Malone, Toad of Toad Hall, and a lot of choral singing. I was getting on the stage at every opportunity for musicals and straight theatre. Opera was a world I knew nothing about. I did pop and jazz. Then I went to Trinity College, Cambridge, to study music and sing in the chapel. I heard about the film auditions from my boyfriend, Andrew Tortis, a tenor.

"The casting director said 'We've got a slot. Somebody has pulled out. We need a photo and audio sample.' We have this log cabin in the garden, and my father and I recorded an aria together on a cassette with him on the piano." A month later her mobile rang while she was walking to evensong at Trinity. "I had just had a lecture. It was dark, and cold. It was Sarah. I'd got the part. It was amazing."

Branagh said: "When I first saw her, she was very winning, with transparent intelligence. Apart from being very beautiful, with sex appeal, she's a terrific listener."

The director said she seemed ideal for a part that required passion and impetuosity to overcome "what you might call the plot challenges". Branagh said he and James Conlon, the conductor, "were very stringent" about the kind of actress they wanted.

Last night a major record company contacted the filmmakers to express interest in Miss Carson.