Polar bears have become a protected species listed as threatened under the US Endangered Species Act as global warming destroys their habitat.

However, the apparent victory for environmentalists has been tempered by a warning that they should not try to use the ruling to rein in economic activity. The green lobby said that the declaration had been delayed to allow oil drilling in the Arctic to continue unabated.

The landmark case marked the first time that the Bush Administration had singled out climate change as a reason to place an animal under the protection of the Endangered Species Act. Yet it came with a key restriction that dismayed the green lobby: the decision should not be misused to harm the economy and “set backdoor climate policy”.

On one level, the move was a significant victory for environmentalists, who had sued the US Interior Department to make a decision about the status of the Arctic’s most iconic inhabitant. It forced the Bush Administration to concede that global warming was the reason for the bears’ decline.

There are an estimated 20,000 to 25,000 polar bears in the Arctic, but scientists from the US Geological Survey predict that two thirds of the world’s bears will disappear in the next 50 years because of a decline in the Arctic sea ice.

In a stark warning last year, scientists at the National Snow and Ice Data Centre said that the total Arctic ice cover had melted to the lowest level in modern records, and that if melting rates continued, the summer-time Arctic could be ice-free within 80 years.

Dirk Kempthorne, the Interior Secretary, said that because of melting ice, “this in my judgment makes the polar bear a threatened species, one likely to become in danger of extinction in the foreseeable future”. But he added that he did not view the protection as a way to regulate greenhouse gases. Environmentalists vowed a flurry of further lawsuits.

The Bush Administration had said that it would make a decision on the bear’s status by January 9 this year. It only announced the protection after a judge agreed with three environmental groups who sued the Interior Department, ruling that a decision had to be made by May 15.

Some in the green lobby say that the delay of the Administration’s decision was a deliberate ploy to make it easier for oil companies to finalise $2.7 billion (£1.35 billion) in offshore oil leases in the Chukchi Sea, an area that is home to about 20 per cent of the world’s polar bears.

“Had the bear been listed prior to January 9 that lease sale could not have moved forward without some substantial additional review of the impacts to polar bears,” said Kassie Siegel, climate programme director at the Centre for Biological Diversity.

Experts say that the bears are losing weight as their hunting grounds diminish. In northern Canada, females that once averaged 620lb (281kg) are down to 485lb. Scientists also say that bears are drowning during long swims from ice to land, and a rise in cannibalism is another sign of how desperate their search for food has become.

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