Personal trainer Adam Thomas says he's devastated after Hannah allegedly broke up with him to get back together with her ex-boyfriend and bandmate Paul.
Speaking to the Sunday People, he claims Hannah kicked him out of their shared flat "out of the blue".
Adam, 35, said: "Hannah moaned to me until she was blue in the face about Paul; she said he was so fat he could hardly dance.
"And now she's with him. He'd moved into my bed before it'd got cold. That's the biggest kick in the teeth."
Adam, who wanted to marry Hannah and is now seeking counselling for his heartbreak, claims "it was a matter of weeks before pictures of Hannah and Paul began popping up" on their shared iCloud account.
He and Hannah fell in love in 2013 when he began giving her personal training sessions, and they got engaged seven months later.
Hannah, 34, and Paul, 38, were a couple for five years during S Club 7's reign at the top of the charts.
They split in 2006 amid allegations she'd cheated on him with Primeval co-star Andrew Lee Potts, who she later became engaged to.
S Club 7 reunited last year and announced a comeback tour.
Their Children In Need performance in November sent Twitter into meltdown as Paul, Hannah, Jon Lee, Bradley McIntosh, Rachel Stevens, Tina Barrett and Jo O'Meara performed a medley of their greatest hits for the first time in 12 years.
When appearing on Loose Women last year, Hannah joked that it "was really awkward" working with Paul again.
For exclusive competitions and amazing prizes visit Reveal's Facebook page NOW!