The One Show host has been prescribed painkillers and will be wearing outfits with sports bras sewn inside for the remainder of her time on the BBC contest.
"Alex has got a gorgeous figure but big boobs do cause back ache," a source told The Sun. "Her back is really bad at the moment.
"She's been suffering for a few weeks now but she is embarrassed and doesn't want the sympathy vote."
Jones last week revealed that she was "in absolute agony" after bruising her ribs during rehearsals with partner James Jordan.
Watch Alex Jones on Strictly Come Dancing below:
Her Strictly co-star Chelsee Healey claimed to be suffering from similar issues ahead of tonight's semi-final, writing on Twitter: "My back is in bits can't walk never mind dance!
"I'm in agony with it, just need to rest up tonight. Still really sore, got a heat pack on it."
Strictly Come Dancing continues this evening (December 10) at 6.30pm on BBC One.
> Wardrobe malfunction leaves 'Strictly' Chelsee Healey in tears
Watch Chelsee Healey on Strictly Come Dancing below: