Sarah Ferguson, the Duchess of York, was not one of the 1,900 family and friends on the guest list.
Her daughters, Princess Beatrice, 22, and Princess Eugenie, 20, first cousins of princes William and Harry, are expected to attend.
The Duchess has also turned down any and all wedding-related offers.
“She has declined all commercial work relating to the wedding,” a spokesman said in People.
The snub did not come as a surprise to the 51-year-old duchess, who last year tried to sell access to ex-hubby Prince Andrew for 500,000 British pounds.
“She never expected to be invited,” her spokesman said.
Fergie has long been estranged from the British Royal Family. In 1992, with her relationship with Prince Andrew already on the rocks, topless photos of her allegedly sucking on an American businessman’s toes appeared in a British tabloid, shattering her image.
The Duchess will be out of the country during April’s ceremony.
She is currently working on a six-part documentary for the Oprah Winfrey Network about her personal struggles with relationships, money, and weight.