Sir Ranulph, 61, made contact via videophone

The 61-year-old's wife Louise told the BBC he had already climbed 2,100 metres back to advanced base camp - a journey which normally takes two days.

But exhaustion and health problems mean he has been forced to travel parts of the journey on his hands and knees.

He had been trying to become the first Briton to reach both poles and conquer the world's highest mountain.

He was an hour into the final 10-hour climb to the summit when breathing diffculties brought on by a bout of bronchitis forced him to pull out.

Speaking from the couple's Somerset home, Lady Fiennes said: "He is delighted to have made it so far back, but absolutely exhausted.

Everybody is very proud of him for making such a wise decision - in my eyes that decision is more admirable than getting to the summit itself. Lady Louise Fiennes

"In parts he had to crawl over small hills he would normally have no problems with - he said he hoped nobody had seen him, but he had to use his hands too."

She said her husband hoped to make it to base camp, at an altitude of 5,400 metres, on Sunday, and to fly home at the end of the week.

"I am happier now he is back down where the atmosphere is less damaging and he can start to recuperate.

"But apparently he has lost a lot of weight, so I may have to fill in the cattle grid at the top of the drive so he doesn't fall through it when he gets home," she joked.

"What he has done is a tremendous achievement. Everybody is very proud of him for making such a wise decision - in my eyes that decision is more admirable than getting to the summit itself."

Sir Ranulph had a heart attack in 2003 followed by bypass surgery, but months later broke records by completing seven marathons on seven continents in seven days.

The explorer is famous for a three-year transglobe expedition - the first successful circumnavigation of the world on its polar axis - which was completed in 1982.