Indian business magnate Mukesh Ambani is the 18th richest person in the world with a personal wealth of $ 24.7 billion in 2012, according to the Bloomberg Billionaires Index, a daily ranking of the world's 100 wealthiest individuals.

Mexican telecommunications magnate Carlos Slim remained the world's richest person last year holding a personal fortune of more than $ 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 $ 21 billion to $ 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 $ 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 $ 60 billion, respectively.

Renowned investor Warren Buffett slipped to fourth place, but added around $ 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 $ 40 billion, putting him in fifth place on Bloomberg's billionaire index.