The 36-year-old star confessed earlier this year that she'd discovered her husband had been having a "full blown sexual affair" with her best friend Jane Pountney and she's now revealed that they first got together while she was in hospital after having an emergency Caesarean section when her waters broke prematurely.
Katie, Kieran, her children - Harvey, 12, nine-year-old Junior and seven-year-old Princess - and Jane and her husband Derrick were all on holiday in France when she first suspected something was going on between them and she now claims they took advantage of her being in hospital.
She told The Sun newspaper: "They had their first sex in the bathroom upstairs in the villa. Later on, they had sex in my bed while I was lying in hospital alone.
"I felt so scared. All I wanted was for Kieran to visit me but he kept saying his shoulder was hurting and he needed to rest. I couldn't reach Kieran on his phone and kept texting Jane asking to tell Kieran to come and visit but she was not passing the messages on.
"Instead, she was telling him things like, 'You're just one of many, Katie will get rid of you soon'."
Katie - who is now heavily pregnant with her and Kieran's second child - had had a traumatic experience giving birth to Jett but claims her spouse rarely came to the hospital to see if they were OK, and instead continued his affair, even after she had been discharged.
She added: "Apparently, I once returned to the villa just 10 minutes after they finished having sex in the kitchen ... I felt like I had lost the man I'd married. Deep inside, I was really struggling. He had dramatically changed and now I know why."