The Xtra Factor presenter, who has enjoyed an on-off relationship with Harry Styles - 15 years her junior - was apparently mortified when she arrived at London's Hammersmith Apollo to see fans brandishing placards about her.
A source told The Mirror: "Caroline was spotted within minutes of entering the venue. The mainly female audience started pointing and jeering, and making a few unsavoury comments - although a lot of the younger fans were being really sweet."
A group of fans were said to be holding a banner which read "Flack off Caroline", while a merchandise stall is reported to have sold all 150 of its whistles by encouraging fans to "whistle if you hate Caroline Flack".
Caroline and Harry, 32 and 17 respectively, were thought to have ended their relationship a couple of weeks ago - but there has been speculation that they are still together after she was reportedly spotted wearing his bracelet this week.
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