The 35-year-old TV personality married the cage fighter in February 2010 however they separated in January of the following year, and the brunette beauty saw the end in sight after he injected his stepson's growth hormones.
She told UK's OK! magazine: ''That was the start of the end. I knew it was over from then on but did not admit it to myself right away.''
Alex claims it was an accident as his prescription of hormones which to help him heal after his fights had been placed next to Harvey's medication in the fridge but Katie is adamant that she didn't keep her eldest child's medication near his.
She added: ''I think people have a measure of Alex when he tried to claim he had taken it by accident. He definitely didn't mix up the medicines because Harvey's were all in a white box with his name on it and are not kept near any other medication.
''He thought I got the medication for free and that it didn't matter that he took it. That sums the 'man' up, taking a sick child's medicine. Things went downhill after that.''
Katie has since married Kieran Hayler in January with whom she had baby boy Jett Riviera in August.
The brunette beauty also has 11-year-old son Harvey from her relationship with Dwight Yorke, and son Junior, eight, and daughter Princess Tiaamii, six, with her ex-husband Peter Andre.
His new album Underneath It All is out now.