As a show of British military finery, the Trooping of the Colour has had no equal for 258 years. Today, the Guards regiments will parade for the monarch and tourists in their spotless regalia, topped with their lustrous bearskins.

But as the Queen takes the salute to mark her 80th birthday celebrations this morning, some 4,000 miles away a group of hunters will be preparing to go into the dense woodlands of Canada and kill an Ursus americanus or Canadian black bear.

If the animal is fortunate, it will die quickly from a shot to the head before having its head and paws severed as trophies and the skin sold to a fur auctioneer. In the past five years alone, 494 of those pelts have been sold at a cost of £321,000 to a long-standing customer of the Canadian fur industry - the British Ministry of Defence.

Yesterday, the MoD - and by default the guardsmen who will stand to attention on Horse Guards Parade today - were accused by animal welfare campaigners of perpetuating the annual slaughter of 10,000 Canadian black bears, many of whom die in pain from botched kills by fee-paying trophy " sportsmen".

The Army confirmed yesterday it would continue to buy between 50 and 100 bearskins a year after it declared a trial to replace the distinctive headwear with hats fashioned from synthetic fur had failed because they got " waterlogged" on rainy days.

Senior officers complained that the man-made hats lacked the "life" and "bounce" of real bearskin - the focus of millions of tourist photographs every year and, as one MoD official put it, "as much an icon of Britishness as a red telephone box".

People for the Ethical Treatment of Animals (Peta), which has waged a three-year campaign to persuade the Army to switch to man-made bearskins and co-operated in the trial, said the MoD was engaging in a cynical "PR exercise" by insisting the synthetic hats were not fit for purpose.

The row has provoked an Early Day Motion signed by 175 MPs calling on the Army to switch to man-made bearskins.

Anita Singh, campaign co-ordinator for Peta, said: "The taxpayer should know that hundreds of thousands of pounds of public money is being spent to perpetuate this shameless slaughter.

"The MoD declared 20 years ago it was willing to switch to a synthetic fur. But, despite being an organisation with the resources to wage modern warfare, it seems incapable of sourcing a suitable man-made material. The result is the Labour Government, which banned fox hunting and fur farming in the UK, is indulging in a trade that inflicts terrible cruelty on wild animals solely for their skins. The MoD has blood on its hands."

The 16-inch high bearskins, worn by the soldiers of the five Guards regiments, have been part of British military uniform since the early 18th century. They are bought on behalf of the Army from unnamed licensed fur traders in Canada. The hats, fashioned from almost an entire bear pelt around a bamboo frame, are made in England by two military outfitters.

The MoD, which pays an average of £650 for a bearskin, said its requirement for fresh hats represented only a tiny proportion - between 0.5 per cent and 1 per cent - of the total number of Canadian black bears killed each year. It is refurbishing a large number of the existing 2,500 hats to reduce demand.

A spokesman said: "None of these bears are killed specifically to make the caps. They are the result of a controlled kill overseen by the Canadian authorities.

"We have made it clear for some time that if we could find an effective alternative to the bearskins we would use it. We are very aware of the sensitivities about the use of fur. But so far there has not been a synthetic fur which performs the task."

With an estimated population of one million, the Canadian black bear is not an endangered species and the annual kill of 10,000 is controlled by a licensing scheme under which hunters pay to kill one or two animals at a time.

The species is listed under Cites, the international register of protected species, to ensure it is not passed off as its endangered relative, the Asian brown bear.

But Peta, which says it has sourced a synthetic fur that does not retain water but the MoD refuses to submit it to an independent laboratory test, insisted the British Army was legitimising an unnecessary and often cruel sport.

Ms Singh said: "There is no official cull in Canada. This is a commercial enterprise in which the female bear, when it has cubs, is the most prized because her fur is thicker and glossier. The fur of the mother bear is used in the bearskins of Guards officers.

"When the mother bear is killed, all too often her young cubs are left to starve. So, for every bear killed, another two or three will also die. Bears are also badly injured and manage to escape only to die in agony."

The armed forces minister, Adam Ingram, this week insisted that the "manner" in which the bears were killed was a matter for the Canadian authorities.