The Xtra Factor host is alleged to have fumed about being kept apart from the One Direction star at Sony's Brit Awards afterparty in Mayfair on Tuesday (February 21).
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According to the Daily Star, Flack was heard complaining: "Why aren't I allowed to speak to him? Why can't I speak to him?"
It has also been claimed that organisers demanded that a picture of Flack and Styles together at the event be deleted from a photographer's camera.
"Caroline wasn't on the guest list. A friend must have got her in," a source claimed. "The One Direction camp weren't thrilled when they saw her. They were not keen for an ex-lovers' tiff to distract from the celebration."
The pair confirmed the end of their short-lived relationship last month, with Styles writing on Twitter: "This was a mutual decision. She is one of the kindest, sweetest people I know. Please respect that."
The romance attracted criticism due to the age gap between 32-year-old Flack and Styles, who was 17 while they were dating.
> One Direction's Harry Styles: 'I'm through with older women'
Watch One Direction in the video for 'One Thing' below:
