Robbie Williams installed "24-hour security" over fears about alien life contacting him.
The Let Me Entertain You hitmaker insisted he has been scared to communicate with extra-terrestrials in case they spoke back to him, and it made him take measures by introducing round-the-clock security at his home.
Speaking to Jo Wood on the Alien Nation podcast, he explained: "The reason why I started having 24-hour security was not for my own safety, it was just because I was scared of what was happening around me when I was alone.
"I was always scared to talk to them because that would make it real and if it was just in my head then it would just be in my head.
"If I said it out loud then I would be more scared than I already was."
The 45-year-old singer added he might have a "mental illness", but admitted he was only saying that to appease any "sceptics".
He said: "Maybe this is a mental illness. I don't think so, but I have to put that out there for the ¬sceptics."
Williams has also claimed he was once haunted by the spirit of Mama Cass.
He rented a Los Angeles property from actor Dan Akroyd 18 years ago but felt terrified as soon as he set foot in the house, which was once home to the Mamas and the Papas singer, who died in 1974 aged 32.
He said: "It was completely and utterly haunted".
The Angels hitmaker spoke directly to the spirit he sensed and said: "I know that you are here and I am going to respect your space. I please ask you to respect mine as I am scared of you."
One evening, The Mamas and the Papas' hit single California Dreamin' came on the TV and Robbie told how the atmosphere in the room changed.
He added: "There was this silence I've never experienced before or since."