Lewis, chairman of Foschini Group, and Kitty were first reported to be dating in 2018, while he proposed in December 2019.
The pair are believed to have met through a mutual friend. Kitty - whose parents are Diana's brother Earl Spencer and model Victoria Lockwood - is believed to be close to Michael's three grown-up children.
Earlier this year, Kitty revealed she was looking forward to settling down into a "really happy" home life.
She said: "I just look forward to a really happy home life: a happy marriage and happy children. And I feel very at peace that things play out the way they should."
However, she explained that she tries to keep her relationship private. She said: "It's not that I don't place an importance on love. It's because, for me, love is the most important thing in the world, and I will protect it with all my heart by not making it a topic of
discussion. You open yourself up to other people having opinions, and it just isn't relevant to anybody's life but mine and my family's."
