John Farnham's cancer battle has left him unable to sing

Writing in his upcoming memoir, 'The Voice Inside', which has been previewed in the Sydney Morning Herald, John explained: "My facial disfigurement from the surgery means I can't open my mouth wide enough for a strip of spaghetti, let alone to sing.

"I can't get the movement to make the sounds I want to make, that's where the vibrations and my voice come from. It's a very disconcerting thing. And trying hurts."

However, he is hopeful that he will be able to sing again in the future.

He wrote: "I was given a gift and to be able to get out there and affect people in some way was special, I would like to continue doing that. Though I am not putting all my hopes into it, we'll see."

John also revealed that he believes his devastating oral cancer was caused by his previous smoking habit.

He said: "Cancer doesn't discriminate, but as soon as I was told the results, I couldn't help thinking it was my own fault, I smoked very heavily all my life."

Meanwhile, speaking previously about his book, John said: "I don't enjoy talking about myself, I really don't.

"Don't get me wrong, I'm an egomaniac, but dredging up the past is just not something I've ever really enjoyed.

"I'll try and share as much as I can, but that's not easy because I've never really been that open. I guess there are reasons for that. Reasons for my reluctance."