It looks like the Princess of Wales, Kate Middleton, wore a black dress by Emilia Wickstead for today’s lunch with the Governor-Generals at Buckingham Palace. It’s known as the Jorgie style.
You can see the dress is gathered into the waist and features a long, flowing skirt.
Emilia Wickstead appears to have sold the ready-to-wear dress in four colours: yellow, navy, pink and bright blue. It’s from a previous season (Pre-Fall 2020) and no longer available to buy. It looks like Kate has probably commissioned a bespoke version of the dress in black. I wouldn’t rule out that she’s wearing the garment in the navy, though.
The ladies at UFO No More note that Princess Theodora of Greece and Denmark wore the sky blue versopn of the to The Wedding of Prince Philippos and Nina Flohr in Athens in 2021.