Caroline Flack
The Xtra Factor presenter alerted police after a threatening fan (image pixellated) sent her a menacing photo on Twitter

TV star Caroline Flack has alerted police after she received an online knife threat.

The Xtra Factor presenter, who had a brief fling with One Direction singer Harry Styles late last year, was sent a menacing photo on Twitter of a wide-eyed teenage girl holding a kitchen knife.

The 32-year-old star re-tweeted the disturbing image to the Metropolitan Police, adding the caption: "Acceptable tweet?"

Flack received a number of abusive messages from One Direction fans when she began dating 18-year-old Styles in November.

Just days ago a 39-year-old man was arrested after allegedly stalking her for more than three months.

He is said to have bombarded her with online messages and repeatedly turned up at her workplace.

A source told the Sun: "Caroline has been very worried. The man didn't seem like a normal fan and her people took it very seriously."