Gauntlett and fellow alpinist James Atkinson, both 21, died on a climbing route on Mont Blanc (photo of the mountain), which at 15,781 feet (4,810 meters) is the highest mountain in the Alps, according to the United Kingdom's Foreign Office.
Gauntlett had become one of the youngest Britons to scale Mount Everest when he conquered the peak at 19 in 2006. He was also known for completing a 26,000-mile (41,842-kilometer) journey from the North Pole to the South Pole in 2007 (watch video of the feat).
British officials said Saturday they believed an avalanche was the culprit. But a Foreign Office spokesperson, speaking on condition of anonymity in line with ministry policy, said Sunday the cause of the accident has not yet been determined.
Gauntlett's mother, Nicola Gauntlett, said, "We don't know exactly what happened, but there was obviously a big fall and they both died.
"We are all just devastated. He's far too young to die," she said.
On the Adventure magazine blog, Editor in Chief John Rasmus said Gauntlett "was a great guy—full of life and confidence and good humor.
"He would have had an amazing life ahead, I'm sure, but the one consolation I can extract is that he was really enjoying the present, and how he was living his life, and he appreciated every aspect of it."
Anna Gregory of the British Mountaineering Council said Gauntlett had been admired by his fellow climbers.
"It was extraordinary when he did Everest," Gregory said. "He was obviously a unique character and an inspiration."
(The National Geographic Society owns both National Geographic News and National Geographic Adventure.)