Richard Walker had been running alongside Iceland colleague Simon Felstead to raise money for the charity Alzheimer's Research UK.

In a post on Instagram, the 43-year-old said he had collapsed and fallen unconscious less than two miles from the finish line with hyperthermic shock, adding that his body temperature was "42C and rising".

"Truth be told, I didn't train enough and pushed way too hard on the day - probably a little bit of Everest arrogance," he said, referring to his successful climb of the world's highest mountain last year.

"I came to about half an hour later surrounded by an amazing team of medics, covered in ice with all sorts of things stuck in me.

"I can't thank them enough - they were the true heroes of the day and I owe them my life," he said of the St John Ambulance workers.

Walker had been running his first marathon but climbed Mount Everest in May last year to raise more than £1m for The National Brain Appeal to build the world's first Rare Dementia Support Centre.

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He said: "I'm so totally in awe of everyone who ran the marathon, and all the inspiring stories behind people's motivation for doing so.

"Seeing all the charity efforts was just so emotional."

More than 50,000 people ran the 26.2-mile London Marathon on Saturday, including politicians, actors in character and amateurs in costume.