Explorer Sir Ranulph Fiennes momentarily fell asleep at the wheel of his car and crashed into a vehicle on the opposite side of the road, a court heard today.
He was left dazed and bloodied by the head-on collision in Stockport, Greater Manchester, but the other motorist was in intensive care for a month with numerous injuries.
The Polar explorer had earlier competed in the 42-mile High Peak Marathon in Derbyshire on the day of the incident in March.
Appearing at Stockport Magistrates Court, Sir Ranulph, 66, pleaded guilty to driving without due care and attention.
He was fined £1,000 and received four penalty points on his licence.
The oldest Briton to reach the summit of Mount Everest was returning from the endurance run in his Jaguar X-type when the crash happened on the A6 Buxton Road in Heaviley.
His vehicle swerved on to the opposite carriageway and hit a Nissan Micra driven by Luigi Castaldo, who was travelling with his 10-year-old son.
The force of the collision shunted the Micra backwards into another car and it then flipped into the air and landed on its roof, the court was told.
Mr Castaldo, who was not wearing a seatbelt, was flung from the driver's side window and was trapped between the roof and the road surface.
His son was wearing a seatbelt and was uninjured.
Sir Ranulph's Jaguar continued forward for several yards and hit the front of another car on the opposite side of the road.
Prosecutor Eddie Coyle said the explorer was "greatly concerned" for the welfare of Mr Castaldo and rushed to him with others in an attempt to free him.
Mr Castaldo went on to spend a month in intensive care and was artificially ventilated for 17 days.
He 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.
Representing himself in court, Sir Ranulph said: "I plead guilty. I hugely regret the incident to Luigi and his family."
He told the bench that he had never lost his concentration in this way previously while driving.
"I travel 40,000 to 45,000 miles a year and it has not happened," he added.
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."
Outside court, he repeated his apologies as he told reporters: "I am really really sorry for what happened to Luigi and his family."
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 charity race, the court heard.
The renowned adventurer was the first man to visit both the North and South Poles by foot and the first to completely cross Antarctica on foot.
Last May, at the age of 65, he climbed to the top of Everest, the world's highest peak, to raise money for Marie Curie Cancer Care.
Sir Ranulph had a double heart bypass in 2003 and suffered a heart attack two years later as he came agonisingly close to the summit of Everest.
He also ran seven marathons on seven continents in seven days in 2003 and climbed the treacherous north face of the Eiger in 2007.
Mr Coyle 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."
The driver of a Ford Focus which was behind the Micra suffered a head injury but no-one else was injured in the crash, which took place at about 1.40pm on March 6 - the day before the explorer's birthday.
The prosecutor said Mr Castaldo spent two months in hospital and then was off work for several more.
He had lost some mobility in his left knee and needs crutches to walk. He was also on medication for his pain and has suffered depression.
"His once active, busy family life has been completely altered," he said.
"The prosecution rules this was not a case of dangerous driving," he added. "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.
A letter from the chief executive of the Marie Curie Foundation was handed to the bench on his behalf.
In reaching its decision, chairman of the bench Kate Beavis said: "We feel the sentence is proportionate in all the circumstances.
"We do not think that a discretionary qualification is appropriate."
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.
AA president Edmund King said: "This case reminds us that we are all vulnerable to falling asleep at the wheel. Drivers need to be aware that sleep deprivation or physical exertion can affect their driving.
"It is estimated that some 20% of fatalities on motorways and dual carriageways might be caused by drivers falling asleep at the wheel.
"Too many drivers take the comfortable modern car for granted and don't pay enough attention to their own vulnerabilities."
Sir Ranulph had arrived early for his appearance and, as the court was locked up, a police officer arrived and escorted him to the nearby police station ahead of the hearing.