John Wilson’s painstaking dedication to restoring long-lost scores written by the greats of Broadway and Hollywood has become legendary – it once took him an entire day to re-create four seconds’ worth of music from The Wizard of Oz – but he’s fed up with discussing it. “It’s boring but it’s necessary, and that’s all I’m prepared to say about it,” he declares.

But inevitably he’s been preparing for this weekend’s Gershwin Prom with his customary fastidiousness. “With a composer as talented as George Gershwin and a lyricist as gifted as Ira, you have two separate careers and an abundance of choice.”

The music will span a 20-year period, covering the brothers’ work from the 1937 Fred Astaire and Ginger Rogers movie Shall We Dance to 1957’s Funny Face, in which Astaire starred with Audrey Hepburn.

“We’re presenting the songs in the treatment that was given to them by the movie studios,” he says, “because in many cases they were either written for the movies or, in the case of George Gershwin, after he died the movie versions became the definitive versions of those songs.”

Wilson includes the Gershwins in the “Big Six” names of great American songwriters, alongside Cole Porter, Harold Arlen, Jerome Kern, Irving Berlin and Richard Rodgers.

While their writing partnership became one of the marvels of the entertainment industry, each of them enjoyed successful collaborations elsewhere. In 1919, when barely out of his teens, George scored his first big hit, Swanee (made famous by Al Jolson), which he wrote with lyricist Irving Caesar.

He worked on Broadway musicals in the early 1920s with William Daly, and wrote the jazz opera Blue Monday with Buddy DeSylva. This was in effect a trial run for the vastly more successful Porgy and Bess, which the Gershwin brothers created in 1934.

“George died [of a brain tumour] in 1937, and Ira went on for nearly five more decades – he died in 1983,” says Wilson.

Each had very different personalities. “George was a bit of a party animal and Ira was bookish and reclusive. He never got over George’s death but did do some marvellous work with other people, including Kurt Weill, Harold Arlen and Jerome Kern. So I felt it was really important to bring some of that into the Prom to give it a different focus.

“For instance The Man That Got Away, which he wrote with Harold Arlen for the 1954 movie A Star Is Born, is one of the greatest achievements in song – a miracle of construction, form and content. I think the short collaboration between Ira Gershwin and Harold Arlen produced a handful of the most important songs ever written.”

Wilson says that George would have developed into a major classical composer had he lived longer.

“On the strength of Porgy and Bess he was a first-rate serious composer,” he says. “He had another opera planned. He’d written a string quartet that he played on the piano to Harold Arlen but never wrote down, and he planned a violin concerto. He was a composer who could orchestrate and write in long forms.”

Nonetheless Wilson, who regularly conducts classical music, strives to break down any divide between classical and popular music.

“I like to think of Schubert’s songs as creating complete little worlds that you can climb inside, but the same happens with a very fine song by the Gershwins or these other important theatre composers. It’s music that has proved its worth and will be around for ever.” 

Prom 38: The John Wilson Orchestra performs Gershwin is live from 7.50pm on BBC Radio 3