Davina McCall has told how her late sister - and God - gave her the strength to get through an exhausting and agonising effort to swim across Lake Windermere.
The presenter's body was limp with her head lolling as she was pulled from the water after clinging on to her support team's kayak to complete her 1.5 mile crossing during a Sport Relief challenge.
Organisers of the seven-day challenge assured fans that McCall - who was crying and struggling to breathe during the swim - was in no danger as she made her way across the lake.
And afterwards, McCall explained that thoughts of her half-sister Caroline Baday - who died from cancer in 2012 - pulled her through.
The presenter's pain and suffering was etched across her face as she had difficulty managing anything other than a doggy paddle and was unable to put her head in the water.
McCall, 46, said: "I imagined my sister holding me and pulling me through it."
And she had also hoped for divine intervention to get her through, admitting: "I got so desperate I looked up to God - if there was anyone up there I needed their help."
However she insisted she did not feel unsafe as she crossed the 5C (41F) lake, which is 183ft (56 metres) deep, accompanied by her trainer Professor Greg Whyte.
Viewers saw her brave the water live on BBC1 this morning, sobbing and red-faced before even taking the plunge. And as her crossing went on she looked increasingly distressed and worn-out.
When she eventually reached her target, her flopping body was lifted from the water and carried to a nearby hotel as the charity's team warmed her up.
Minutes later she was seen smiling and about to get her hands around a mug of hot chocolate in a photograph posted on Twitter, with her core temperature on the way back up.
And before long she was on a bike for a 65-mile leg in her attempt to travel 500 miles under her own steam as she heads from Edinburgh to London by running, swimming and cycling.
She said of her lake crossing: "The moment I got in the water, my chest felt crushed - I couldn't do front crawl, I couldn't put my head under water, I couldn't breathe," McCall said.
"One thing Greg told me that was pulling me through was to think of the building ahead of us as a magnet, drawing us towards it.
"I kept trying to lift my arm to swim, trying to pull the crawl out of the bag, but I couldn't even get it up. I ended up doing doggy paddle.
" It was such a struggle, but I have such a great team around me, I couldn't have done it without them. Although I'm shattered I never felt unsafe, I'm in the best possible hands."
Prof Whyte said: "I was in the water throughout the whole swim to ensure Davina's safety. It was very tough and very cold, but I can reassure people that she was safe throughout and never in any real danger. I'm full of admiration for her determination to finish this swim and get back on her bike."
Within hours of setting off on Saturday morning her team was concerned she was exhibiting signs of hypothermia as she battled severe weather during a tough 130-mile stint in the saddle.
But after being examined she was cleared to continue with her journey, the BT Sport Relief Challenge: Davina - Beyond Breaking Point.
Yesterday she scaled Scafell Pike, where she again faced treacherous weather, trudging through snow and negotiating her way through the cold mist.
In previous years brave celebrities have put themselves through other draining pursuits for Sport Relief. David Walliams picked up stomach bugs during his swim down the River Thames, as well as developing sores from his wetsuit, and two years ago John Bishop was left in agony amid worries about stress fractures as he did back-to-back marathon distance runs en route from Paris to London.