The builder and stripper, 27, says that Katie walked in on him cheating on her and also reveals that Katie has been "a mess" since she found out.
Except for one tweet aimed directly at Katie on her 36th birthday (which she didn't respond to), this is the first time Kieran has spoken up since the discovery of his betrayal.
In an interview with today's Sun On Sunday, Kieran admits he had a seven-month affair with Katie's best friend Jane - and even had unprotected sex because it "added to the risk".
He also says he slept with Jane on New Year's Eve at Katie's stables before returning home and climbing into bed with his wife.
He says: "I am going to try and save my marriage but I've almost ruined it. Kate has been an absolute mess. She is focused purely on her children and pregnancy - I have no idea if she will forgive me."
He also reveals that the affair was eventually discovered by Katie during a family holiday in Cape Verde in April, where Jane and husband Derek were also present.
Pregnant Katie, already mum to four children, found the pair kissing on a sunbed according to Kieran.
He says: "Kate came from nowhere. All I heard was Katie calling [Jane] a sl*t and a wh*re."
Last month, 36-year-old Katie announced on Twitter that she was divorcing cheating Kieran. She said: "No human being deserves this, especially being six months pregnant."
Katie is pregnant with their second child together - a baby girl.
Kieran admits his affair with Jane, who is also married, started when their first son Jett was just two months old.
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