The singer songwriter – who found fame in the Stereo Kicks band on the ITV show and has penned tracks for acts including JLS and Ronan Keating – revealed the news on Monday in a long Instagram post.
He posted the tribute alongside a picture of 34-year-old Hampson cradling their eight-month-old son, saying she died in the early hours of Saturday.
Mann, 28, said: “I can’t believe I am writing these words but my darling Dani – my best friend, my everything and more, the love of my life – passed away in the early hours of Saturday morning, 18th June.
“On what was supposed to be the happiest day of our lives ended in irreversible heartbreak.
“I feel like I have cried an ocean. We never made it to the alter; or got to say our vows, or dance our first dance, but I know you know that you were my entire world and the best thing that has ever happened to me, Danielle.
“I will wear this ring that I was always supposed to wear as a sign of my unconditional love to you.”
He told of how he was planning to raise their son in her memory: “I am completely broken trying to process this and I honestly don’t know where to go from here, but I do know I need to use any strength I can muster for our little boy.
“I will not be a mark on the parent that you had already become but I promise I will do my everything to raise Bowie just the way we always wanted. I promise you he will know how amazing his mummy was. I promise to make you so so proud.
“The most beautiful person inside and out. The most incredible soul. We have lost such a special person and I am sure we are about to see an abundant outpouring of love that reflects that.
“I will try to find peace in your messages and comments, but right now I am grieving and I will be for a very, very long time.
“My darling Dani, the brightest light in any room, my world is nothing but darkness without you. I will miss you forever.”
His famous friends flooded social media to offer condolences.
Among them was singer Lewis Capaldi, 25, who said: “Love you brother,.”
Girls Aloud’s Nadine Coyle, 37, said: “Sending so much love, light and healing to you, Bowie and everyone who loved Dani!”