The 34-year-old beauty starred as judge on the show in 2009, before she was unceremoniously dumped by the 'Britain's Got Talent' judge, who mistakenly thought she was a singer.
In her new book - which is being serialised in The Sun newspaper - Kelly explained: ''Half way through filming he said: 'You're a singer aren't you, Kelly?'
'''No,' I replied.
'''Are you sure?' 'Absolutely.' ''
What's more, Kelly said her exit from the popular ITV show came about in abrupt fashion, leaving her seething with anger.
She said: ''I left at the end of the third day, then my agent rang me and said: 'I don't know how to say this but they don't want you back.'
''I was angry and hurt, and the first thing I said is not printable!''
Kelly explained she received a call from Simon a few days later, when he explained he'd been under pressure to oust her from the show and that ''some people had been jealous''.
She added: ''There is an expression that comes to mind - be careful what you wish for. And honestly, what you shouldn't ever wish for is a telephone call from Simon Cowell.''
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