It will be the ultimate expedition - the first circumnavigation of the Earth's surface over both poles.

After seven years of meticulous planning and arduous training under the patronage of Prince Charles, a british team of young amateur adventurers led by a 35-year-old swashbuckling baronet is to leave Greenwich, England, at the beginning of September for a three-year journey along the Greenwich Meridian through desert heat and polar cold.

They will travel by Land-Rover through Europe and the west African Sahara, by ship to the Antarctic, by snowmobile across 2,600 miles of frigid Antarctica to the South Pole, by boat across the Pacific to Alaska, by motorized rubber raft up the Yukon and Mackenzie Rivers and through the icy, 3,000-mile Northwest Passage, and by ski and snowmobile over the Arctic ice cap to the North Pole before returning by ship to Greenwich.

They will use the most sophisticated space-age equipment to cope with the dangerous terrain and temperature extremes of 120 degrees above to 120 degrees below zero Fahrenheit. They will spend the dark polar winters in igloo-like cardboard huts, cross snow and ice on snowmobiles with pontoons to keep them from sinking if they fall through, walk across water and slush with British-invented watershoes, bridge glacial crevasses and desert sand rifts with portable aluminum bridges, and secure gear, while wearing heavy gloves, with ropes that fasten without knots.

They will map at least 800 miles of previously unexplored terrain in the Antarctic, carry out extensive glacier, weather and magnetic field research in both polar regions, and scientifically monitor the effects of the journey on their own bodies, while wearing miniature tape machines to record the strain on their hearts. They and their equipment will be shown off along the way at exhibitions in eight cities in Europe, Africa, Australia and America.

The Transglobe Expedition is a quintessentially British adventure in the tradition of David Livingstone, who explored Africa in the mid-1800s, Capt. Robert Falcon Scott, who reached the South Pole just a month after Norwegian Roald Amundsen in 1912, Sir Edmund Hillary, who conquered Mount Everest in 1953 and Sir Francis Chichester, who completed his solo sailing journey around the world in 1967.

It is also hoped, as Prince Charles has pointed out, that it will boost British prestige and demonstrate the British ingenuity that went into many of the gadgets they will use.

The leader of the Transglobe Expedition is Sir Ranulph Twistleton-Wykenham Fiennes, the third baronet of Banbury, born after his father died of wounds fighting as a lieutenant colonel in the Royal Scots Greys in Italy in 1943. Educated at Eton, Fiennes served as an officer in his father's regiment and in the sultan of Muscat's Armed Forces in Oman, winning the Sultan's Bravery Medal in the Dhofar campaign in the Arabian Desert which he filmed.

After leaving the army, Fiennes spent his time leading British expeditions to the White Nile, the Jostedalsbre Glacier and the Headless Valley in British Columbia. He supported himself by writing books and giving lectures about these adventures. He talked companies into providing him the equipment he needed for his expeditions by agreeing to exhibit it at trade fairs.

"I got born with a title, but the money got lost somewhere along the way," Fiennes, an engaging, lanky, rawboned man, said while demonstrating the expedition's watershoes in St. Katherine's Dock next to the Tower of London and Tower Bridge on the Thames.

His demonstration of the watershoes - which were invented by a Royal Navy officer, look like two miniature canoes and are maneuvered in the water like cross country skis are on snow - was one of the highlights of a recent dress rehearsal here of the exhibitions he will stage on his way around the world.

"There was no way we could make this expedition without sponsorship," Fiennes said. In return for promotion at the trade exhibitions, more than 300 sponsoring companies have provided the Transglobe Expedition everything from a Twin Otter airplane and 213-foot, 1240-ton polar exploration ship to snowmobiles, one-man rubber boats, navigational and scientific measurement equipment, tropical and arctic clothing, and even cooking utensils, stoves and food.

Fiennes said the expedition was conceived by his wife, a shy 32-year-old woman known formally as Lady Virginia Fiennes but called Ginny by the men she will accompany on the journey. She will run the polar base camps for the Arctic and Antarctic crossings, directing communications and logistics.

Fiennes said he was bored after finishing his book on their Headless Valley expedition, so his wife "thought up an expedition that would take us through various countries we wanted to see."

In 1972, they began plotting the route, seeking sponsors and picking "a small tough team of volunteers" willing to give up everything else for the decade it would take to prepare and carry out the expedition. The team eventually numbered seven - three, including Ranulph Fiennes, in the "ice group" to make the long, lonely, dangerous polar treks, two including Virginia Fiennes to man the polar base camps in sub-zero temperatures, a pilot for the airplanes to parachute supplies to the expedition, and a captain for the 12-man crew of the ship to transport the equipment and carry the expedition team on 52,000 miles of ocean voyages.

The second man in the ice group, 33-year-old Oliver Shepherd, a former sales manager for a London brewery, met Fiennes when they both were running in the Welsh 3,000, a gruelling 26-mile marathon up and down the Cairngorm Mountains in Wales.

"anyone who applied for the expedition after that," Shepherd said, "had to run the race."

The third man, Charles Burton, 36, a South African who served in the British Army and later worked for a british security firm before going back to South Africa to start a small business, met Fiennes and Shepherd at a dinner party here. He had just returned to London after his business failed.

"When they told me what they were doing, it appealed to me," Burton said. "I didn't have anything else to do."

They supported themselves during the years of planning and training by working in London pubs. None of them had polar experience, which is required by the Royal Geographical Society, British Foreign Office and the other countries controlling exploration of the polar regions.

"They pointed out that no privately mounted expedition had been allowed to visit Antarctica since the war," Fiennes recalled. "We, they said, were amateurs. We would get lost and become an embarrassment, and have to be rescued.

"You really have to know your beans in the Antarctica because search and rescue costs so much. So we had to prove ourselves."

Testing equipment they had already collected from a number of sponsors, Fiennes' team trained in the Snowdonia Peaks of the Cairngorms in Wales and on the Greenland Icecap 600 miles north of the Arctic Circle, where conditions are similar to Antactica.

Then, with Virginia Fiennes running the base camp on the edge of the polar ice, they journeyed from February through May of 1977 across 960 miles of the frozen Arctic Ocean to within 160 miles of the North Pole before an early summer thaw began breaking up the ice and forced them back.

Only five expeditions in history had gotten that for before them. Most had used dog sleds. Fiennes, Shepherd and Burton tried out their specially designed snowmobiles pulling sledges carrying supplies.

In a report to the Royal Geographical Society, they described their progress across the seemingly mountainous ice in temperatures down to 60 below Fahrenheit: "For the first 120 miles, progress was slow and painful. Massive ridging of the ice floes presented walls every 600 yards to be cut through with axes...." Snowmobiles and sledges were hauled by hand over walls of jagged ice up to 37 feet high. "This caused heavy perspiration that later turned into ice particles inside the underwear."

When it was over, they had earned the approval of polar explorer Walter Herbert for the Royal Geographic Society and they had won a royal patron.

"I had gone to Buckingham Palace one day and left a letter for Prince Charles containing details of our expedition," Fiennes recalled. "There was no immediate answer and I had almost forgotten about it. Then, when we were within 200 miles from the North Pole, we got a message from London via our base camp. He was backing us."

They found sponsors for nearly all their equipment and money to buy or rent anything they might need in an emergency along the way. The polar exploration ship, for example, was donated by the Bowring shipping, trading and finance firm, which had backed British polar expeditions dating back to Scott's voyages to the Antarctic. Even the insurance against search and rescue costs was donated.

While final details are being sorted out before Prince Charles flies down from Balmoral Castle to see them off at Greenwich, the team continues its physical training. Burton, who has a bit of a bartender's paunch, pointed out that they do not want to get too fit.

"You can't lose too much weight going in," he said, "because you need body fat to lose over the polar winter. We each lost about 30 pounds in the Arctic."

Each also has taught himself a number of practical and technical skills, from rudimentary medicine to auto mechanics to running scientific tests.

"We are totally interchangeable," Shepherd said. "If someone were to get killed, we would take over his functions immediately. Then somebody from the base camp would be flown in to replace him, and a backup here in London would be sent to the base camp.

"It is quite likely that someone will get killed, actually," he added, "although I don't want to dramatize it. I'm not really frightened. I've been through it once now, and I respect the cold."

Fiennes noted that "we have not been to the Antarctic before" and nobody has preceded them along the first 800 miles of their journey there. It is expected to be a particularly arduous climb up steep, uncharted terrain to an 8,000-foot plateau. "If there are crevasses," he said, "it could be very unpleasant.

"In the Arctic," he added, "we will be in difficulties unless we get it right all the way."

The journey in one-man motorized rubber boats through the Northwest Passage, Burton explained, "will be a fight against time. The ice opens up a narrow passage for only a very short time. We must make 80 miles every day."

They will be aided, however, by the ingenious design of their rubber boats, which have metal skids on them so they can skim across the top of interfering ice patches. And for icy slush, there are the watershoes, although Fiennes fell backward into the brackish water of St. Katherines' Dock while demonstrating them.

"One would be foolish to say one is not worried," said Virginia Fiennes, the most reserved of a very low-key group. we have been planning this for over six years, and we have given it a great deal of thought." CAPTION: Map, Route of the Transglobe Expedition, By Dave Cook and Richard Furno - The Washington Post; Picture 1, This ship will be used to make certain segments of the circumnavigation of the world.; Picture 2, Members of the expedition: From left, Oliver Shepherd, Charles Burton, Virginia Fiennes, Sir Ranulph Fiennes and Anthony Bowring. They plan to start their three-year journey along the Greenwich Meridian in September. Photos by John Benton-Harris for the Washington Post