“We’ve been waiting for this for 80 years,” said Rahman recently in The Hindu, the Indian daily newspaper. “I think it will create a bridge between Western and Indian audiences, and lead to more people exploring Indian music.”

While Rahman first came to the attention of Western audiences with his film music for Elizabeth: The Golden Age (2007) and stage shows Bombay Dreams and The Lord of the Rings, his already stellar profile will go into overdrive should he win. Numerous Hollywood offers have been coming his way since the film’s release.

Just as in a Bollywood film, in Slumdog Millionaire the music is almost as important as the imagery and the narrative, and is central to its ambience of grit and grandeur. “We wanted it edgy, upfront. Danny [Boyle, who directed] wanted it loud,” says Rahman.

Until now, the 43-year-old Rahman was often seen as a kind of Indian Andrew Lloyd Webber, inventive and full of melodic facility, often veering into the middle of the road. Lloyd Webber compares Rahman to Paul McCartney.

What makes the soundtrack something genuinely new is the collaboration of rappers such as Blaaze (also based in Chennai), and more particularly the presence of Sri Lankan/British musician MIA, who co-wrote the Oscar-nominated song O Saya, and whose huge global YouTube hit Paper Planes is also on the soundtrack. She brings some Lennon-like acidity to Rahman’s McCartneyish sweetness.

Paper Plane, with its percussive gunshots, Clash sample and lyrics about visas and bombs in the voice of an immigrant sandwich-seller went top 10 in the US and was the first song to top both the alternative and hip hop charts there. When the song became a hit, MIA said: “The other songs on the chart are Katy Perry and the Jonas Brothers. Then you see Paper Planes and it’s cool because there’s hope. Thank God the future’s here.”

The combination of Rahman and MIA superbly complements the film’s mix of urban realism and fairytale in its gritty but epic score, with gorgeous tunes riding on the dust of the slums. The score seems utterly contemporary, its globalised sounds reflecting a world in which a man of Kenyan ancestry is now in the White House. Forget about economic or cultural decoupling – we are all in this together, and need inspiring moral tales for uplift.

Rahman describes MIA, who shares a Tamil family background, as “a real powerhouse”. “Somebody played me her CD, and I thought, ‘Who is this girl?’ She came here [MIA recorded her last album partly at Rahman's studio in Chennai] and knew all my work, had followed my work for ages. I said, ‘Cut this “my idol” crap. You have to teach me.’ ”

It was MIA who pressed for both songs to be nominated. “I knew Jai Ho was a hit, and I thought we’d push for one song, but MIA thought we should push for O Saya, too. And she proved to be absolutely right.”

Rahman’s first breakthrough tune Chinna Chinna Aasai, in the 1992 film Roja was a song of a poor man dreaming of riches. Rahman’s family had struggled after his father, also a composer, died when he was nine and the family were reduced to hiring out instruments. His father’s eclectic tastes was inherited by his son, who formed a rock band in his teens and studied Western classical music (at Trinity College in London) and Indian classical music.

Since Roja, he has written scores of hit movie soundtracks for films such as Lagaan, Dil Se and Guru. Whether or not the published figures that he has sold more than 100 million CDs and 200 million cassettes are accurate (with piracy endemic in India it’s anyone’s guess how many he has sold), he is already one of the most successful musicians on the planet.

“Compared with other Indian film composers, I only write about six movies a year. Others write up to 60,” he tells me in the unlikely setting of his house in Tufnell Park, where he lives when he is in London.

He is softly spoken and surprisingly modest about his accomplishments, ascribing his talent and luck to divine intervention: “I believe that whatever comes at a particular time is a blessing from God.”