The British supermodel accepted a marriage proposal from The Kills' frontman on Tuesday night (01.02.11) while the pair were in bed, and she is delighted the pair have made this commitment to each other because she believes they are "soul mates".
A friend of the couple told The Sun newspaper: "Kate is delighted. Jamie is the man for her - they are soul mates. She just wanted to make him work for her hand in marriage.
"He has been badgering her to get married but she wanted to take her time. Over the last year they have grown so close, they are like man and wife."
Jamie - who was introduced to Kate by mutual friend Sadie Frost in September 2007 - proposed with a vintage ring from the 1920s, and even asked Kate's parents Linda and Peter for permission beforehand.
The pair are now ready to start planning their wedding and have decided on July 2 as a possible date, and the 37-year-old catwalk legend is also thinking up ideas for a "huge party" to celebrate after the ceremony.
The friend added: "They're going to have the mother of all weddings. It will be a huge party, one of the biggest she has every thrown."
She has an eight-year-old daughter, Lila Grace, from a romance with magazine publisher Jefferson Hack.