Lady Kitty Spencer, 29, is to wed fashion tycoon Michael Lewis, 60

Lady Kitty Spencer is seen leaving Michael Kors Collection Spring 2018 Runway Show. Photo / Getty Images.

Princess Diana's stunning niece is set to marry a fashion tycoon five years older than her father.

The Daily Mail report 29-year-old Lady Kitty will wed South Africa-born multi-millionaire, Michael Lewis, who is set to turn 61 this month, to make him 32-years-older than his second wife-to-be.

Lewis is also five years older than Lady Kitty's father, Earl Spencer.

"Michael proposed to Kitty before Christmas," one of her friends told the Daily Mail.

"She's been in Cape Town for the holidays and told her mother and the rest of the family. Michael is loved by all of them. Despite his wealth, he's very humble and low key."

Lewis previously married a woman named Leola in 1985 and has three adult children.

Lady Kitty Spencer. Photo / Getty Images.

Lady Kitty was spotted kissing Lewis last August, as the pair were leaving Club 55 in St Tropez.

The couple first went public with their relationship when they were sighted together strolling through New York City in May.

Kitty told the Daily Mail at the time that she wasn't in any hurry to start a family.

"I can see myself having children at some point," reported the Daily Mail.

"But I'm only in my 20s. I do have friends my age who are having children, and they are so, so, sweet, but I think everyone should do things in their own time."

Lady Kitty's last boyfriend was Italian property magnate Niccolo Barattieri di San Pietro.