Heat is on Sir Ranulph in marathon
Sir Ranulph Fiennes faces the most punishing stage of the notorious Marathon des Sables challenge - with a 56-mile slog ahead.
The 71-year-old explorer is coping with temperatures of more than 50C (122F) and endless sand dunes as he attempts to become the oldest Briton to complete the six-day marathon, dubbed the toughest footrace on Earth.
Having already covered more than 100 kilometres (62 miles) in the last three days, Sir Ranulph is feeling the effects of the Moroccan heat, particularly on his heart following two previous heart attacks and undergoing a double bypass.
Sir Ranulph 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."
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 Marathon des Sables in Morocco covers 251 kilometres (155 miles) including the distance of a marathon a day, with runners carrying everything they need on their backs.
But today the racers will face the equivalent of more than two marathons, and is expected to take Sir Ranulph more than 20 hours to complete.
Sir Ranulph received a message from the Prince of Wales wishing him the very best of luck before taking part in the race.
He hopes to raise £1 million for terminal illness care charity Marie Curie.