Knut the polar bear, who became world famous after he was hand-reared by keepers after his mother rejected him, has died at the age of four.

A spokesman for Berlin zoo, where Knut was kept, said the bear was found floating in the pool inside his enclosure.

Bear keeper Heiner Kloes said the cause of death was not known, and a post-mortem examination would be held.

In captivity, polar bears can live until they are about 30 years old.

Berlin Mayor Klaus Wowereit called Knut's death awful.

"We all held him so dearly. He was the star of the Berlin zoos," Mr Wowereit told BZ newspaper.

Knut became world-famous after his birth in 2006, as news of his unusual upbringing emerged.

In 2007, Knut generated more than 5m (£4.4m; $7m) euros in extra income for Berlin zoo, from the sale of tickets and Knut-branded merchandise.

As his fame grew, a row over the royalties from the bear's popularity erupted between Berlin Zoo and Neumuenster Zoo, where he was born.

Eventually Berlin Zoo agreed to pay 430,000 euros to compensate Neumuenster.

German polar bear Knut drowned when he fell into a pool of water in his enclosure, after collapsing due to swelling of the brain, experts say.