Wearing nothing more than a pair of trunks, goggles and swimming cap, Mr Pugh - otherwise known as The Ice Bear - had to hack into the ice covering much of the fjord with a pickaxe in order to enter the lake and complete the three-quarter mile (1.2km) swim.
The 36-year-old solicitor - who is already the first person to have carried out a long distance swim in the Antarctic and the Arctic - lost feeling in his hands and feet 15 minutes after plunging into the lake, overlooked by the Jostedalsbreen glacier, in northern Norway.
After emerging, Mr Pugh, reportedly suffering from a brief bout of hypothermia, admitted he "took his body to the edge" by swimming for as long as he had.
He said: "I'm ecstatic. It was the hardest cold-water swim I have ever done because the water is so fresh. After many years of training I can control the initial shock, but it took my body to the edge swimming for that long."
Scientists are fascinated by Mr Pugh's ability to endure temperatures that would induce hypothermia in most people within minutes. With the average swimming pool temperature at about 27C, and the English Channel - distinctly chilly for most people's tastes - is generally around 17C or 18C, scientists say many people would find their bodies disabled if they attempted the swims Mr Pugh has completed. The normal response to being suddenly immersed in icy water is cold shock - causing involuntary gasping for air, and the rapid loss of muscular control and co-ordination. Sports scientists carrying out a study of Mr Pugh's reactions to his polar swims believe he has either the conscious or unconscious ability to raise the core temperature of his body in the minutes before he enters the water.
In January, Mr Pugh, who was born in Britain but moved to South Africa when he was 10, became the first person to have completed long distance swims in all five oceans. The feat was the culmination of 14 years of long-distance swimming, beginning in 1992, when he crossed the English Channel. In 2003, he broke the world record for the most northern long-distance swim after enduring a 5km swim around North Cape, in the Arctic, Europe's most northern point, and last year Mr Pugh swam for 1km in the Barents Sea off the island of Spitzbergen, 1,100km from the North Pole, in temperatures of -3C.
Four months later, he broke the world record for the southern-most, long-distance swim when he swam 1km around Deception Island at the tip of the Antarctic Peninsula. He then crossed Nelson Mandela Bay in South Africa, completing the journey with a 17km swim from Manly Beach to the Sydney Opera House in January 2006.
Mr Pugh's swims are the subject of a study led by Professor Tim Noakes of the University of Cape Town. Speaking after he completed his swim off Deception Island, Professor Noakes said that Mr Pugh's core body temperature rose to 38.2C (from 37C) before immersing himself in the ocean, a reaction known as anticipatory thermogenesis - the creation of heat before an event. He called the response "fascinating" and said: "I believe it's a Pavlovian response after years of cold-water training. It gives Lewis the edge and he is therefore able to swim in water which would disable most humans within seconds."
The professor said Mr Pugh's swims had helped collect data which illustrated the narrow margins for survival in such cold water.Reuse content