The 62-year-old presenter was diagnosed with the disease - which has also spread to his bones - a few months ago, but has now chosen to speak out in a bid to encourage others to get tested.
It comes ahead of his appearance on this week’s ‘The Great Celebrity Bake Off for Stand Up to Cancer’, which he filmed not knowing he had the disease.
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.
“Maybe if I’d got it earlier and stopped it at the prostate, I’d be in a much better state,” he says.
“The GP said, ‘It’s clear you have prostate cancer and that it’s spread to the bone.’ And all of a sudden you’re in this dark chasm.”
The Classic FM presenter added that he spent a “bleak weekend” revealing the news to his family - wife Sesi and his three children, Henry, 29, Will, 28 and Flora, 26.
“Those first few days were probably the worst days of my life,” he says.
“The worst thing is, you carry it through the day and then you go to bed at night and wake up in the morning and it comes to you again. I have got cancer. I’ve still got cancer. It wasn’t a bad dream. And that takes a lot of dealing with.”
He goes on to reveal that although “the contract I thought I had with life has been shortened quite a bit”, he still has a life expectancy of 10-14 years.
“I wouldn’t have any complaints,” he says. “The only thing is, I don’t want the rest of my life to be defined by the illness.
“You have to be positive, don’t you? I know I’m not going to get cured and I’m realistic about the long-term prospects, but they’re not bad.
“Most importantly, I really do think I’ve had a wonderful life, with amazing experiences as a reporter and a presenter. And if it was all to end tomorrow, I couldn’t have any complaints. I’ve had a really marvellous time.”
‘The Great Celebrity Bake Off For Stand Up To Cancer’ airs on Channel 4 on Tuesday 6 March.
Read the full interview with Bill in this week’s Radio Times, out now. www.radiotimes.com