Canadian soldiers fighting in southern Afghanistan are three times as likely to be killed as British troops and four times more likely to die than American GIs, according to a report by the Centre for Policy Alternatives, an independent think-tank.

The claim has also been supported by a separate study for Canada's Department for National Defence, which said its country's servicemen were "at significantly higher risk" in Kandahar than UK troops in neighbouring Helmand or US forces along the Pakistan border to the north-east.

The casualty toll has now become a major political issue which could topple Canada's minority government.

Ottawa has threatened to end its military involvement in Afghanistan from next January unless other Nato countries deploy at least 1000 combat troops and transport helicopters.

Ottawa has a 2500-strong contingent in the Taliban heartland, but only 1000 of them are fighting soldiers, with the others in supporting and supply-chain roles.

Canada's military death toll stands at 78 from its front-line units, compared with 87 British soldiers from a "bayonet strength" of more than 3000 and a total garrison of 7800. The Americans have lost 415 soldiers from a total of 28,000.

Since the US-led invasion to topple the Taliban regime in 2001, a total of 698 Nato, US and allied soldiers have died.

The biggest contingent in the country is American, while the smallest ranges from Austria with three soldiers to Georgia with just one token representative in uniform.