Television presenter Davina McCall has described how her arduous fundraising exploits last month left her in a "very dark mental place".

The 46-year-old, who was the face of Channel 4's Big Brother coverage for a decade, suffered an emotional meltdown during her charity exploits for BT Sport Relief Challenge: Davina - Beyond Breaking Point.

The mother-of-three embarked on a week-long 500-mile run, cycle and swim from Edinburgh to London, raising more than £750,000 for good causes and shedding plenty of tears along the way.

At one point, television viewers saw Davina being pulled out of the water as she completed a 1.5-mile swim across Lake Windermere.

In an interview with the Guardian's Weekend magazine, Davina said the sheer physical exertion resulted in her "sobbing my guts out".

She said: "It was like somebody had opened the floodgates and I let out every single bit of grief that I had.

"I kept crying every day... I think I was just so tired that I had no defences, and I was just sobbing at everything. "

She added: "I'm normally a chipper person. If you're in a positive state of mind you can cope with pain and jolly yourself along, but I had moments when I went into a very dark mental place."

The former heroin addict admitted she doubted herself throughout the challenge, but persevered after seeing the work her charity ordeal would help fund at a Sport Relief project in Africa before setting off.

She also said the memory of her half-sister, Caroline, kept her going. Caroline died in 2012 after being diagnosed with cancer.

Davina, who also spoke frankly about her difficult relationship with her mother, said: "She (Caroline) was our other half, she was my husband's sister, and it was the hugest loss of my life. I felt like she was with me (on the challenge) a lot."

Sport Relief is due to take part next weekend. For more information or to donate, visit www.sportrelief.com.