“Everyone dreams of winning an Oscar,” he says. “It gave my work a new level of recognition and legitimacy.” Rahman’s gongs, for best song and best score, made him only the third Indian to win an Academy award. The success of Slumdog Millionare brought other advantages – “I had the chance to meet some of my great heroes,” says Rahman. “I got to meet Barbra Streisand and work with Celine Dion, and I was the first Indian to perform at the Hollywood Bowl.”

Today we’re a long way from Los Angeles, in his north London base, a house near Hampstead Heath. Rahman has been visiting and working in the UK for the last 15 years, and later this month will attend the Southbank Centre’s Alchemy Festival (“exploring the culture of India, its diaspora and its relationship to the UK today”), at which the London Philharmonic Orchestra will perform some of his best-known works – from his Oscar-winning soundtrack of course, but also from the likes of Elizabeth: the Golden Age, the hit musical Bollywood Dreams, and some of his landmark Indian films, such as Lagaan and Jaane Tu … Ya Jaane Na.

Rahman may have only achieved global fame recently, but he has been making music for most of his life. He was born to a Hindu-Tamil family, in which his father was a composer, arranger and conductor for Malayalam movies – those made in the Indian state of Kerala, in the Malayalam language, which are considered more serious and realistic than Bollywood films.

“I started playing music at the age of five,” he says, “the piano and harmonium, and after my father died when I was nine my mother was determined that I was going to also be a musician.” How did he feel about his mother’s ambition? “It wasn’t as plain to me that I would be a musician,” he says, laughing, “but I also knew that I had a talent for it.”

Rahman recalls listening to western music such as Jim Reeves and the Carpenters alongside the work of Indian film composers including Naushad Ali, Madan Mohan and Roshan (who wrote in Hindi), and Tamil composers such as Vishwanatiian Ramamurthy and KV Mahadevan. He formed a rock band in his teens and went on to study western classical music in London at Trinity College of Music before beginning his musical career back in India writing advertising jingles. His breakthrough came when he scored the 1992 Tamil movie Roja. It was a hit, and Rahman’s soundtrack led to him winning the Indian national award for best music composer.

Rahman’s great innovation for Indian movies was to introduce orchestral melodies to the traditional Bollywood soundtrack’s fondness for violent, slashing violins and dramatic tablas. This earned him comparisons to Andrew Lloyd Webber and Paul McCartney. “In India we love melodies in the background of scenes,” he says, “but in the west there is a sense that soundtracks should not distract so there is a greater preference for more ambient sounds and plain chords.”

Indian cinema was once the preserve of a largely south Asian audience. Rahman has been fortunate to work in an age in which Indian films have become more global affairs. Not only are they now seen around the world, they are also made around the world. Bollywood films are now routinely shot in the US and Europe, and western stars – including Snoop Dogg, Akon and Kylie Minogue – have put in appearances. The songs, once so quintessentially Indian, now sometimes sound almost indistinguishable from western pop and dance music.

The Indian films I watched as a young boy featured the songs of such immortals as Lata, Mohammed Rafi and Kishore Kumar and they could never be mistaken for anything other than Indian music – that was their appeal and it gave those of us who listened to them a proprietorial pride that this was “our music”. Is there not a danger now that the success of Indian cinema has come at the price of losing its essence? “When something is new it is overdone,” he says. “When stereophonic sound first came out, people would pan the sound all the time from one speaker to the other but then it settled down to what was necessary for the song. So right now you get Indian films shooting in Europe and America but eventually it will all settle down again.”

And, Rahman says, an international composer cannot make music that is purely national in quality – something he is bearing in mind for his forthcoming London concert. “This will be the first time I am playing in London since winning the Oscar,” he says, “so it is important to play music that will be accepted by an international audience but which retains an essential Indian quality.”

Despite his fame, Rahman stresses the virtues of humility, which he attributes to his conversion to Islam at the age of 23 (at which point he changed his name from Dileep Kumar to Allah Rakha Rahman). “What appealed to me about Islam was that this is a religion based on unconditional love and a belief in one god and one love,” he says, “and I was especially drawn to Sufism which has a rich musical tradition. I never skip prayers. I find it releases me from tension and gives me hope and confidence that Allah is with me, that this is not the only world.”

It his faith, he says, that leads him to feel a duty to use his music to spread what he believes is the true message of Islam. What does he say to the Muslims who say that Islam forbids music? “In that case why is the azan [the call to prayer] in tune?” he asks. “Why is it musical? Islam has been hijacked by the extremists and what drives me in my own work is to create music that will bring people together.” Next week’s concert is part of this mission, an effort to use music to unite. “At one of my concerts you will see people of all colours and religions together. That is what music can do. A song is more powerful than a thousand rallies.”

The London Philharmonic perform the music of AR Rahman at the Royal Festival Hall on 7 April. Box office: 0844 875 0073. Rahman also performs his Jai Ho: The Journey Home concerts in Glasgow, Manchester and London from 23-26 July