Inside the Most Horrific Aspects of David Bowie's Secret Cancer Battle — As Fans Get Set to Mark Music Icon's 10th Death Anniversary

OK! can reveal how David Bowie kept his 18-month battle with liver cancer hidden as he recorded his final album, Blackstar.

The iconic singer-songwriter, who died at age 69 in January 2016, is remembered in the upcoming documentary Bowie: The Final Act – which is set to be released just ahead of the 10th anniversary of his passing – and we can report it contains moving footage of his longtime producer Tony Visconti recalling the moment he realized the horrific scale of Bowie's illness.

Visconti, 81, who produced 14 albums for Bowie over five decades, including Heroes, Young Americans, Diamond Dogs, Ashes to Ashes and Fashion, describes the first time he saw his friend visibly affected by cancer on the show, which is due out January 3.

He recalls: "I remember going to his offices, and a girl who worked there opened the door, and she had been crying. "Her eyes were red. She led me into this small room with a small table and a seat on either side and says, 'David will be with you in a minute.'"

He continued: "David then sits in front of me and he has got a woolly cap on. The first thing I notice is that he has got no eyebrows. I went, 'I know what that is. It is chemotherapy.' I cried my eyes out."

Visconti also vividly remembers the emotional exchange that followed. "He is my lifetime friend since the '60s," he added. "I started mumbling things like, 'Don't worry, we will get through this. We will get the best doctors in the world. You're strong. You are going to get over this.'"

"I just started blabbering s--- like that and he comes to my side of the table and he starts hugging me. Given what he knew about his own health, I think he wanted to make the best album of his life."

According to Visconti, Bowie revealed his illness to the musicians working on Blackstar in New York before recording. "Before he started the album he said to everyone, 'Guys, I have something to tell you.'"

"He pulls his cap off and he stands there bald in front of them and they are looking at each other like, 'Who wants to run away? Who wants to crawl under their piano? What the f--- do you say?' The song 'I Can't Give Everything Away' on the album makes me very tearful. He is talking about his disease."

Visconti's recollections highlight both Bowie's privacy and professionalism. He also says on the show: "Musicians were sworn to secrecy as they worked on Blackstar. (Bowie) didn't want the focus on his health – he wanted the focus on the music."

Keyboardist Jason Lindner, who also worked on Blackstar, says on the documentary: "We were all pretty nervous and on edge to meet David Bowie. We stood up when he walked in, almost like soldiers at attention. He was immediately like, 'Guys, just sit down, relax.' David told us in the beginning, 'I don't know what this is going to be. We will just make some great music.'"

The documentary also charts Bowie's career from his early beginnings, his Ziggy Stardust breakthrough, mainstream 1980s success, the '90s wilderness years, and his return to form at Glastonbury 2000, headlining 29 years after his debut at the festival. It also details his final months, illustrating how the singer concealed his illness while completing his artistic legacy.

Bowie: The Final Act will screen in selected cinemas from Boxing Day and air on Channel 4 on January 3.