The couple have turned to pastry chef and food writer Claire Ptak, owner of the small Violet Bakery in east London, to make the cake, which will be covered with buttercream and decorated with fresh flowers.
Rather than a traditional rich fruit cake, the bride and groom-to-be have opted for a lighter cake, which Kensington Palace said they were very much looking forward to sharing with guests at their reception.
American former actress Ms Markle, who used to run her own lifestyle website thetig.com, previously interviewed Ms Ptak for her blog.
Ms Ptak said she was thrilled to be chosen to bake the royal wedding cake, saying she shared the couple’s values on “food provenance, sustainability, seasonality and most importantly flavour”.
California-raised Ms Ptak formerly worked as a pastry chef for chef and food activist Alice Waters at Chez Panisse in Berkeley, California.
On moving to London she worked at The Anchor and Hope, a gastro pub near Waterloo Station famed for food, and staged at St John and Moro restaurants.
She started her own business at a stall in Broadway Market, east London, cooking from home, before opening her organic bakery in 2010.
“I can’t tell you how delighted I am to be chosen to make Prince Harry and Ms Markle’s wedding cake,” the chef said.
“Knowing that they really share the same values as I do about food provenance, sustainability, seasonality and most importantly flavour, makes this the most exciting event to be a part of.”
Ms Ptak is the author of the Violet Bakery Cookbook (Square Peg). The Duke and Duchess of Cambridge had two cakes made for their 2011 nuptials. The first was an eight-tiered iced fruit cake featuring the “language of flowers” made by Leicestershire baker Fiona Cairns. Their second cake was a childhood favourite of William’s - an unbaked chocolate biscuit cake.