Also revealed are the menu for the royal wedding reception and news that Harry’s new wife Meghan Markle will make a speech at the event.
The lunchtime reception – which will be followed by an evening reception at 7pm – is being hosted by the Queen at St George’s Hall in Windsor Castle, with about 600 guests invited.
Elton John is among those present and has given a performance.
Approximately 7,500 items of food were prepared by a team of 25 chefs, led by royal chef Mark Flanagan.
Among the culinary delights on the menu are Scottish langoustines wrapped in smoked salmon with citrus creme fraiche and croquette of confit Windsor lamb, roasted vegetables and shallot jam.
Sweet canapes include champagne and pistachio macaroons, orange creme brulee tartlets and miniature rhubarb crumble tartlets.
The wedding cake, designed by Claire Ptak, features elderflower syrup made at the Queen’s residence in Sandringham from the estate’s elderflower trees, as well as a light sponge cake made especially for the couple.
A filling made from Amalfi lemon curd and elderflower buttercream ties all the elements together.
The cake is decorated with Swiss meringue buttercream and 150 fresh flowers, mainly British and in season, including peonies and roses.
Speaking about the cake on Friday, Ms Ptak said: “It’s a non-traditional layout. It’s a slight shift from tradition.”
She described the flavour of the cake as a balance of sweet and tart, coupled with an “ethereal” elderflower, adding that the texture is “really lovely and the flavour is quintessentially spring and British”.
Californian Ms Ptak spent three years working for Michelin-starred influential American chef Alice Waters before coming to the UK. She is thought to have met Ms Markle when the actress interviewed her for her blog.
The cakemaker, whose Violet Bakery is based in east London, has been working in the kitchens at Buckingham Palace for the last five days.
The separate parts were transported to Windsor Castle where it was assembled in situ on Saturday morning, before the flowers were added.
To drink, guests will be served Pol Roger Brut Reserve non-vintage champagne, a selection of wines and soft drinks including an apple and elderflower mocktail, made with the same elderflower syrup as used in the wedding cake, and Sandringham Cox’s apple juice.
The reception will include the cutting of the wedding cake and speeches from the Prince of Wales, Prince Harry and Meghan Markle.
The Duke of Cambridge, who was the best man at the wedding ceremony, will act as compere for the reception.
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:: See all of the guests as they arrived at the royal wedding – use our exclusive, interactive Who’s Who feature on the Sky News app and website.
:: Watch a special highlights programme in ultra high definition at 9pm on Sky News.