The 62-year-old was one of four celebrities taking part in a special edition of the Channel 4 show in aid of Stand Up To Cancer.
The presenter filmed the charity special in September - two months before he was diagnosed with incurable prostate and bone cancer.
Speaking at the end of the episode, Bill said: “I was getting pain in my legs and in my hips particularly, and I thought, ‘This is old age’.
“Eventually the pain got so bad I thought I’d better go see my GP. He said, ‘I’m just going to give you a blood test, an MOT if you like, to check a few things out’.
“The next morning he called me and asked me in pretty quickly. And the doctor said, ‘it’s fairly clear you have advanced prostate cancer and it’s spread to the bone.’
“Now I have to deal with the future or having much less of the future than I thought I’d have...it’s a very, very difficult moment.”
He continued: “Those first few days after diagnosis are really horrible, probably the worst few days of my life.
“The love of my family and friends have really kept us afloat.
“I told my daughter and I had to tell my sons on the phone...we all cried. If I’d then test a few years ago, we could have knocked it on the head earlier, my chances of survival would be a lot better.
“Now I’m on a course of chemotherapy. If it was all to end tomorrow I would not feel hard done by. I’ve had an amazing life, I’m incredibly lucky and if one thing comes out of this is that if you know your time is limited you appreciate the world around you so much more and particularly the love of my family.”
Viewers at home were in tears at the emotional video…
In an interview with his friend and former colleague Sian Williams in this week’s Radio Times, Bill reveals he is currently undergoing chemotherapy after the cancer spread to his legs, hips, pelvis and ribs.
Following Tuesday’s screening of the ‘Bake Off’ special, Bill took to Twitter to thanks fans for their love and support.
He wrote: “Totally overwhelmed by the love I’ve had on here today, and deeply moved - to tears on occasion (well I am a bit of a softy).
“Thank you so much to everyone who’s been in touch, it really means a lot. Watch Celeb Bakeoff 8pm, support @StandUp2C - and guys, think of getting tested!”