The supermodel who has been dating The Kills guitarist Jamie Hince since last August is believed to have got engaged during the couple's recent break in Amsterdam.
A beaming Kate reportedly emerged from a bar in the Dutch city and told fans: "I'm getting married!"
One onlooker added to Britain's The Sun newspaper: "The pair were in a bar looking very loved-up. Kate was so excited it looked like she had just accepted his proposal there and then. She was very happy.
"She came out giggling and seemed a little bit drunk. Lots of fans were taking her picture on their mobile phones. She told them she was getting married and when one girl asked her when, she said, 'Soon.'"
Friends of Kate who has a five-year-daughter Lila from a previous relationship - are thrilled she is finally settling down, following her turbulent romance with drug addict Pete Doherty.
One said: "We're all so happy for Kate. Everyone was worried she would end up with Pete, who we thought was no good for her.
"We were convinced that, for some unknown reason, she saw him as the love of her life. Jamie looked like it could have been a bit of a rebound thing at first, but now it seems like it's the real deal."
Kate, 34, split from Babyshambles frontman Pete last June following claims he had cheated on her with a South African model.