Can theatrical lightning strike twice? That is the pressing question surrounding the Gershwins’ Porgy and Bess, to give the full title of a £3.5m musical opening next month, of which one thing is already certain: no competing show has a better score. But its director, Trevor Nunn, has set himself a sizeable challenge.

It is 20 years ago since Nunn electrified Glyndebourne with a production of Porgy and Bess, starring Willard White and conducted by Simon Rattle, that remains stored on this theatregoer’s mental hard drive. Passionate and furious, soaring and soulful, it restored a would-be opera from 1930s America to its proper place in the international canon. Performed across three Glyndebourne seasons, and in a revised form at Covent Garden, it lent a racial diversity to a repertoire that wasn’t used to mixing the Gershwins’ Catfish Row with Mozart and the like.

Now Nunn is revisiting the piece as a West End musical, with the material reconsidered, reorchestrated and trimmed. He roots what he has done in the intentions of its composer, George Gershwin, who died of an inoperable brain tumour in 1937, aged 38, unaware of the considerable journey his creation would go on to make. His older brother Ira, the lyricist, didn’t die until 1983, and was survived by his widow, Leonore, whom Nunn managed to meet three times; she died, in 1991, at 90.

“What I recall,” says Nunn, speaking during rehearsals at the Savoy Theatre, “when I was researching the Glyndebourne production was that although George said it was very important to him that what he was constructing was an opera, he also said it should reach the widest possible audience. He wanted to write an opera that crossed boundaries, not something only for aficionados or for those people who could afford to go to the Metropolitan in New York. Having done Porgy the opera, I was always disappointed that the number of performances was low and the range of the audience narrow. I felt we were fulfilling one half of George’s intention.” Here is his chance to more than fulfil the other half.

The interest for real Porgy buffs lies in a commercial production attempting to find cohesion in a work that was starting to boast more versions of itself than many Shakespeare plays have folios. At its core remains the story drawn from the 1925 novel Porgy, by DuBose Heyward, who also wrote the book and co-wrote the stage show’s lyrics (he had turned Porgy into a 1927 play). Telling of a crippled beggar and his love for the alluring Bess — in turn, in thrall to her pimp, Crown, and a drug dealer called Sportin’ Life — Porgy and Bess makes of the South Carolina slums a world complete in itself; yet its resonances extend well beyond this terrain.

“It’s a classic piece of American theatre,” says Clarke Peters, the American actor-singer, long resident in London, who is taking the male lead. Peters had never seen Porgy and Bess live when he was offered the part. Aware of the controversy that has swirled at times around the show’s spoken and sung patois — white men imagining how the black community might speak — he travelled to South Carolina to do his own research. His findings? “ There are still people who speak like that, in that dialogue,” he says. He now feels the piece “has had a bad rap”. In fact, he argues, “it’s a wonderful marriage of emotion and music, and is psychologically completely true”.

The score has been adapted by Gareth Valentine, the British conductor, composer and musical arranger. One of his main tasks was to rework for a 20-strong orchestra a piece that in the opera house might use one of 80. He also adjusted the keys, so that, for instance, Nicola Hughes’s Bess possesses a new-found earthiness. In the opera, says Valentine, “the text is earthy, but one tends to find that, with a very high soprano or formal baritone sound, it gets conventional and formal”.

Purists, inevitably, will gnash their teeth. They should be aware that this production comes with the full backing of the various Gershwin estates (which were also approached by the director Richard Eyre to do something along these lines: Nunn won out because of his history with the work). “What’s going to happen will be exciting,” says Robert Kimball, the musical theatre historian who advises the Ira and Leonore Gershwin Trust. “I can’t predict other than that. Porgy and Bess has always been poised between those two worlds of musical and opera, both of which Trevor understands.”

Besides, it’s not as if the opera is going away “It’s still there,” Nunn points out, “for anybody to do at any time. What we have is additional for those people who would never think of going to the opera house.”

Porgy and Bess is previewing at the Savoy, WC2; opens Nov 9