The bears have been found with both male and female sexual organs.

The BBC's Richard Wilson investigates in the Norwegian Arctic

Scientists say chemicals used in heavy industry are causing the abnormalities, which could eventually lead to the bear's extinction.

Polar bear birth-rates are already falling, and the Norwegian Polar Institute has detected high levels of PCBs (polychlorinated biphenyls) in the bears' blood.

Scientists say polar bear could be extinct in 50 years

PCBs are industrial chemicals known to damage hormonal systems, causing infertility and sexual deformities. They also attack the immune system, and in high cases can be fatal.

Polar bear research scientist Andrew Derocher said the effects of PCBs on the polar bear could be devastating.

"Even a very small change in reproductive rates can cause a very slow decline in the population level, and if this happens you could see a population greatly reduced or in the worst case go to extinction," he said.

Causes of contamination

Research indicates there are hot spots of PCBs scattered across the Norwegian Arctic.

They are believed to be carried to the Arctic Circle in air and sea currents from Europe and Asia.

Most countries banned PCBs in the 1980s, but once released into the environment they can persist for decades.

Cubs are most at risk

The polar bear suffers because it is at the top of a food chain.

At each step in the food chain, more PCBs accumulate in the body fat.

The polar bear cubs are in the greatest danger because they receive very high doses through their mothers' milk.

Humans could be next

Scientists are worried that humans may also be at risk from PCBs, whose effects are felt much further afield than the Arctic Circle.