Sir Ranulph Fiennes fell asleep at the wheel, crashing head-on into another car, shortly after completing the 42-mile High Peak Marathon in Derbyshire.
Sir Ranulph was driving near Stockport when his Jaguar X-type swerved on to the opposite carriageway and hit a Nissan Micra driven by Luigi Castaldo, who was travelling with his 10-year-old son. Mr Castaldo's car was pushed backwards into another vehicle before it turned over and landed on its roof, a court heard.
Mr Castaldo was thrown from his car and trapped between its roof and the road. He was in intensive care for a month with serious injuries.
Sir Ranulph, 66, was given a £1,000 fine and received four penalty points on his licence after pleading guilty to driving without due care and attention at Stockport magistrates' court.
Speaking outside of the court, he said he was "really, really sorry for what happened to Luigi and his family".
He said that he had rested specifically for the High Peak Marathon with extra sleep beforehand and eating pasta to keep up his energy levels.
"There is no reason why I should have [fallen asleep] when this strange thing happened. I assumed that must have been the case. I travel 40,000 to 45,000 miles a year and it has not happened."
Sir Ranulph is the oldest Briton to reach the summit of Everest and was the first man to get to both the North and South Poles by foot as well as being the first to cross Antarctica on foot.
He was left dazed and bloodied by the crash. Mr Castaldo suffered bruising to the head, a broken collarbone, a fractured right wrist, four broken ribs and damage to his femur. Both his lungs were deflated and his left kneecap was shattered.
He also lost some mobility in his left knee and needs crutches to walk, was on medication for his pain and has suffered depression. "His once active, busy family life has been completely altered," prosecutor Eddie Coyle said.
The driver of the car behind suffered a head injury but Mr Castaldo's son, who was wearing a seatbelt, was unhurt.
Mr Coyle said Sir Ranulph was "greatly concerned" about Mr Castaldo and rushed to try to free him along with others. He said: "Mr Fiennes seems to have suffered a micro-sleep or a momentary loss of concentration. It is not possible to say which it was but it was clearly for a brief duration.
"This was not a case of dangerous driving. This was very much a case of momentary loss of consciousness and concentration. Speed was not an issue, drink was not an issue, the manner of his driving was not an issue.
"The endurance run he had been on the previous night was something he had performed in recent years. He describes feeling elated on the journey back for his team winning and he is, by any measure, no ordinary man in terms of his achievements and stamina.
"Clearly he must have been tired on this journey home but, given these endurance feats, he cannot be judged by the ordinary standards of a normal driver or human being. He has been driving since 17 and drove tanks as part of his Army career. He drives many thousands of miles with his work and his charity work. He is a very, very experienced driver."
Magistrates were told he had six penalty points for speeding offences committed in July 2007 and November 2007, although the former had since dropped off his licence. Kate Beavis, chairman of the bench, said: "We feel the sentence is proportionate in all the circumstances."
Sir Ranulph, of Exford, Somerset, said: "I hugely and totally regret what happened. Obviously it was not personal or intentional." He was also ordered to pay a £15 victim surcharge.
Teams of four compete in the High Peak Marathon to test their fitness, endurance and navigational skills through the night as they tackle terrain which is mostly made up of pathless peat bog.
Sir Ranulph's veterans' team, the Polar Explorers, won the race. Last May, at the age of 65, he climbed Everest to raise money for Marie Curie Cancer Care. He ran seven marathons on seven continents in seven days in 2003 and climbed the north face of the Eiger in 2007.
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