Andrew Ridgeley "cried like [he'd] never cried before" when he learned George Michael had died.
Andrew told the Mail on Sunday newspaper: "When I received the news of my oldest friend’s death on the afternoon of Christmas Day last year, I had, only five minutes beforehand, sent him a message wishing him a wonderful Christmas.
"That night, after I had phoned our friends to convey the dreadful news – and despite having shed an ocean of tears already that day – the sheer eviscerating sense of loss cut my legs from beneath me and I cried like I’d never cried before."
And Andrew admitted his friend's death, which was attributed to natural causes related to a heart condition and fatty liver, was even harder to cope with than the passing of his parents because it was so unexpected.
He said: "Nothing had prepared me for the depth of pain George’s death precipitated.
"The shock and disbelief were overwhelming. I had lost my parents in recent years and yet, this was entirely different, a loss I had not contemplated, a loss that was inconceivable, one so abysmally sad that in that moment I was consumed by it.
"I had always been aware of George’s importance to me, of the bond of friendship and of the sparkle and light, effervescence and electricity that suffused the music we made.
"Yet in the intervening years between our career together as Wham! and where our different lives had subsequently led us, I had somehow lost sight of quite what my childhood best friend meant to me."
Andrew is now backing a campaign to honour George by getting Wham's festive hit 'Last Christmas' - which peaked at number two in 1984 - to number one over the festive season.
He said: "It has only been as we make our way towards Christmas and the anniversary of his passing that I have decided to speak in depth, and to lend my support to the most fitting tribute to my dearest friend of all – a bid to make 'Last Christmas' this year’s Christmas No 1.
"For me and so many others, the bittersweet nature of the song means it will forever mark the best and the worst of times...
"George was passionate about getting his songs to number one, which is why it is important to me now that the anniversary of his death be marked by an accolade of this kind...
"For me and for countless others, this coming Christmas will be tinged with sadness. I hope the haunting beauty of Last Christmas and the pleasure it brings might ease that sorrow – and that, with your help, posterity may one day record Last Christmas as a number one. It would please my old chum no end!"