Mexican telecommunications magnate Carlos Slim remained the world's richest person last year holding a personal fortune of more than USD 70 billion, it said.
Ambani, 55, Chairman & MD of Reliance Industries, has also retained his position as the world's richest Indian for the sixth year in a row and increased his ranking from 19th to 18th position and personal wealth from USD 21 billion to USD 24.7 billion in the world, the Index showed.
The world's top 100 billionaires got even richer in 2012, with their total wealth rising by around 15 per cent to USD 1.81 trillion, it said.
Microsoft founder Bill Gates and the founder of fashion retailer Zara, Amancio Ortega, rounded out the top three spots, with wealth estimated just over and just under USD 60 billion, respectively.
Renowned investor Warren Buffett slipped to fourth place, but added around USD 5 billion to his fortune over the year despite donating a substantial amount to charity last year.
IKEA founder Ingvar Kamprad saw his net worth rising 16.6 per cent to approximately USD 40 billion, putting him in fifth place on Bloomberg's billionaire index.