Jeremy Clarkson has said he thought Richard Hammond was dead after being involved in a serious crash while filming The Grand Tour.
Hammond was driving an electric supercar which crashed and burst into flames after completing a hill climb during filming in Switzerland.
The 47-year-old climbed out of the car before the fire and suffered no serious injury, although he will need surgery to fix a broken leg.
Clarkson told of the "coldness" he felt when he realised the crash had happened to his co-host, adding that his knees turned to "jelly".
He said Hammond was referred to as a "body" following the incident, and that he did not want to see him, "not after a crash that big".
Clarkson wrote on Drive Tribe, a digital platform for car enthusiasts: "I don't know what went wrong. Hopefully, when he comes out of surgery and is feeling up to it, he will be able to tell us.
"What I do know is that I genuinely thought he was dead."
Clarkson said he had finished filming for the day and was waiting for Hammond and fellow presenter James May to complete a hill climb when he learned that a test driver had had an "off" in one of the cars. He mistakenly thought it was a Lamborghini Aventador that had crashed and not the Rimac Concept One that Hammond had been driving.
Initially angered, Clarkson said it quickly became obvious that something serious had happened.
He wrote: "And as I stood there, waiting for news, it dawned on me that the burning car was not yellow, as the Aventador was. It was white. Hammond's Rimac had been white.
"And I can feel it now; the coldness. My knees turning to jelly. It was Hammond who'd crashed.
"I was joined at this point by James who'd arrived on the scene just before me in his Honda NSX. He was in a right old state, his arms waving frantically, his eyes wide. 'Hammond's in there,' he was screaming."
Clarkson added: "Then came news from a nearby marshal that he wasn't. That he'd got out before the fire started. And that 'his body' - that's what they said - was behind a screen at the bottom of the hill.
"I could see the screen. I could see the paramedics behind it. I couldn't see Hammond. I didn't want to see him. Not after a crash that big."
Clarkson said they were then told Hammond was fine by their security man, and that he is sure his co-host will share further details "when the lucky sod feels up to it".
Hammond has apologised to his wife and children and joked that he is "not dead" in a video filmed from his hospital bed.
A spokesman for The Grand Tour thanked paramedics for their "swift response" after the accident, and added that the cause is being investigated.