Mysterious death: Malyshka drowned in her enclosure at Banham Zoo

One of the world's rarest tigers has died in mysterious circumstances at a zoo.

Malyshka, a Siberian tiger which was pregnant with three cubs, drowned in her enclosure at Banham Zoo, near Diss, Norfolk - even though the predators are excellent swimmers.

The five-year-old - mother to 17-month-old cubs Vasya and Kuzma - was found by keepers in shallow water.

She also shared her enclosure with 15-year-old male tiger Mischa, described as 'good natured and placid'.

Zoo director Martin Goymour said a post mortem examination had confirmed Malyshka had drowned, but the cause of the accident was a mystery.

He said: 'Malyshka was such a strong and healthy tigress and showed no signs of external or internal injuries.  

'The pool in the enclosure is not deep or considered hazardous.

Motherless: Malyshka's 17-month-old cubs Vasya and Kuzma

'All the zoo staff, particularly her keepers, are very much saddened by her loss.

'We are professional in what we do in animal care at all times, but something like this hits home.

'We need to understand whether there was any medical condition that may have caused her to drown.'

Malyshka was among the top five most crucial breeding females in Europe.

There are believed to be no more than 450 adult Siberian tigers left in the wild. Tigers in captivity typically live for 16 to 20 years.

Malyshka showed no signs of injury.

Mr Goymour said the zoo would look to bring another female into the enclosure to continue its commitment to the survival of the breed.