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The Croydon-born supermodel gave photographers a flash of the sparkler when she left her hotel in Paris on Thursday.
She was in the French capital to attend a Dior party.
Meanwhile, back in London, Kate's rumoured fiancé Jamie Hince couldn't help but smile as he headed to his car outside the couple's north London home.
He is said to have surprised Kate with a night time proposal, going down on one knee after turning the lights on again after they had got into bed.
The 37-year-old model glammed up to attend the fashion party in the city of light – looking beautiful in a chiffon purple dress.
She teamed it with two tone grey heels and a leather clutch, and a funky stone-coloured fur jacket.
Kate will reportedly tie the knot on July 2, with her daughter Lila acting as bridesmaid.
Forty-year-old Jamie - guitarist with band The Kills – met Kate at one of his gigs in 2007. They were introduced by Sadie Frost and hit it off straight away.
They hit a rocky patch two years ago and split up for three days, but were soon back together.
A deciding factor in Kate agreeing to take the next step was apparently the way Jamie has become a father figure to Lila.