He has just conquered Mount Everest - but Richard Parks' 737 Challenge has been threatened by frostbite.
The former Wales rugby union player, 33, is aiming to become the first man to stand on the seven highest summits of the world and the North and South Poles.
After summiting Everest on Wednesday, Parks discovered frostbite on his right big toe.
There are now "severe doubts" for his next climb at Alaska's Denali mountain.
Parks has already conquered five of the seven summits and both poles as part of his record-breaking quest.
He said the challenge was "a bit like snakes and ladders".
"I had no indication I had frostbite on summit day, in fact I thought I'd bruised my toenail on the front of my boot on the down climb, but on arriving back in base camp and visiting the medical centre, I've just found out that I have frostbite in my right big toe," he said.
Parks is due to be flown by helicopter back to Kathmandu on Saturday to get back to sea level as soon as possible. There are concerns he could lose his toe
"Denali is 50:50 as is losing my toe," he said.
"Jagged Globe, my expedition partner, have ensured I'm being looked after by the very best high altitude medics and I will do everything I can to get through it. I'm shell shocked, gutted and angry".
Jagged Glob director Tom Briggs said: "Richard is getting the very best advice from the top vascular surgeons and experts in cold injuries in the UK as we speak."
Parks has just two more mountains to climb to make history by becoming the first ever person to stand on all 7 summits and 3 poles in the same calendar year.
So far he has conquered the South Pole, Mount Vinson in Antarctica, Aconcagua - the highest summit in South America, Africa's Kilimanjaro, Australasia's highest peak Carstensz Pyramid and The North Pole, where he was joined by double Olympic champion rower Steve Williams.
Parks said: "The summit push couldn't have gone more to plan, although one of the toughest days of my life, it was a genuinely enjoyable climb across the razor sharp balcony ridge, up the mixed climbing of the large rock section, the summit ridge, Hillary step and finally the feeling of stepping onto the summit was out of this world."
The next leg of the challenge is Alaska, where he is set to climb Denali, the highest mountain in North America.
From there he would move on to his final leg of the race to climb the highest summit in Europe, Russia's Mount Elbrus.
Richard, his family, and the 737 Challenge team now face an anxious wait for him to be flown by helicopter back to Kathmandu to see if his toe can recover and the challenge can continue.
Parks' record breaking challenge is aiming to raise £1million pounds for Marie Curie Cancer.