A group of British explorers attempting the first circumnavigation of the earth's surface over both poles have suffered a serious setback 450 miles from the North Pole on the last difficult leg of their three-year journey.

Vital equipment--including five snowmobiles, spare parts, tools and scientific devices--was destroyed by a fire this week in a storage shed at the camp of the Transglobe Expedition at Alert Base in northern Canada. The fire occurred just hours before the snowmobiles were to have been shipped by air to the expedition's leaders.

The fire left Ranulph Fiennes, an Eton-educated baronet, author and adventurer, and Charles Burton, a former soldier and small businessman, on foot pulling heavy sledges over high ridges of ice in temperatures as much as to 40 degrees below zero with only a week's supply of food and other provisions.

Expedition officials in London said they are working feverishly to obtain new equipment to be flown to Fiennes and Burton. They said they located new snowmobiles in Canada today but still have not found spare parts likely needed to repair them on the rugged journey across the ice.

Expedition spokesman Paul Clark said that Fiennes and Burton were willing to continue on foot with a new supply of food and other provisions. But he said they would not be able to reach the North Pole during April before the summer sun begins to break up the polar ice, making passage impossible. They must cover 10 to 20 miles a day, Clark said, but have averaged only six miles a day on foot.

"This is a bad stroke of luck, but that's part of what expedition-making is all about," Clark said. "We know we are in a critical phase, but we are not begloomed."

The expedition set out from Greenwich in September 1979 and is due to return in August after crossing the polar ice cap. It has already crossed the Sahara Desert and Antarctica via the South Pole, and has navigated the dangerous Northwest Passage in Canada. During rest periods in cities in Europe, Africa, Australia and America, it has staged trade fairs of the sophisticated equipment donated by sponsors for travel on land and sea in extremes of heat and cold.