The Sun says that the incriminating message from Jane Pountney read: "Has she left yet. Can't wait to see you."
It's thought that the star, who is six months pregant with her and Kieran's second child, began to have suspicions about her best friend Jane and her hubby while on holiday this April.
According to reports Katie, 35, had found Kieran and Jane 'kissing' on the beach while they were on a recent holiday to Cape Verde.
The clinch is said to have been dismissed as a drunken snog, but Katie was on the alert.
Kieran's feelings for Jane were then confirmed to the glamour model when she reportedly found secret messages on his phone after they got back from holiday.
One Whatsapp message from Jane - who was Katie's bridesmaid - is believed to have read: "Has she left yet. Can't wait to see you."
A source told The Sun on Sunday: "When she got back from holiday she went through the internal CCTV camera footage from inside their home but found nothing incriminating."
But then, she's said to have found the message.
Jane, 49, and her husband Derrick have confirmed her affair with Kieran, and Jane said she "deeply regrets" what happened.
Kieran, 26, is yet to comment.
Katie married Kieran at the beginning of 2013 and only recently spoke of how happy she was in her third marriage.
She said: "He's the right man for me. I'm happy. We're happy. I've never been this happy with anyone before."
Now, heavily pregnant Katie is focussing on her growing baby.
She announced the traumatic news that she plans to divorce Kieran last week.
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