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Sir Ranulph Fiennes tackles 50-degree heat in gruelling six-day Marathon des Sables

Sir Ranulph Fiennes prepares for the Marathon des Sables Photo: Marie Curie/Liz Scarff/PA Wire

The famed explorer Sir Ranulph Fiennes is taking on arguably his most grueling challenge yet - a 56-mile run in the 50-degree heat of the Moroccan desert.

Sir Ranulph, who is 71, is aiming to become the oldest Britain to complete the Marathon des Sables - which is widely considered the toughest footrace in the world.

Sir Ranulph has had two previous heart attacks Credit: Marie Curie/Liz Scarff/PA Wire

Having already run 62 miles across the Moroccan Sahara in three days, today's stage is the most punishing of all, and will see participants running more than double a normal marathon, taking around 20 hours.

Sir Ranulph admits he has begun to feel the effects of the heat, particularly on his heart following two previous heart attacks and undergoing a double bypass.

He said: "As I'm walking I can feel a cadence under the wire where they sewed me up - it serves as a warning that I need to slow down.

"It feels like a rhythm behind my heart, it makes me feel dizzy and that is when I know I need to slow down. I think it is caused by the extra soft sand under foot or the inclines."

The six-day 'ultramarathon' covers 156 miles Credit: Marie Curie/Liz Scarff/PA Wire

Sir Ranulph's trainer, Rory Coleman said: "Even just walking a few yards takes such an extra effort."

Sir Ranulph added: "You think you see the shimmer of a checkpoint but then an hour later you still haven't reached it."

The six-day 'ultramarathon' covers a total distance of 156 miles, the equivalent of one marathon a day, with runners forced to carry everything they need on their backs.

Sir Ranulph is running the race in an attempt to raise £1 million for the terminal illness care charity Marie Curie.