Luke, 18 - who had been friends with the 23-year-old TV presenter for a year before they started dating - confirmed their engagement on his Facebook page.
Sources revealed messages of congratulations had come flooding in from friends and family on the social networking site.
Kelly sparked rumours of an engagement to model Luke when she began wearing a sparkling ring on her wedding finger.
She said in an interview recently: "Would I marry him? Yes, you never know.
"But I'm still really young so we'll have to wait and see. I really do love him, we get on so well."
Kelly 'married' her previous boyfriend Fields rocker Matty Derham in an inflatable church at the Electric Picnic festival in Ireland in September 2006. Her representative later denied the ceremony was legally binding.