The Duchess of York is said to be "deeply unhappy" after being left off the royal family's "inner sanctum" guest list for this month's wedding.
Prince Andrew's ex-wife is among 600 guests invited to the ceremony in St George's Chapel at Windsor Castle on May 19 and to the reception that is being held afterwards in St George's Hall, the Daily Mail reported.
But although Prince Andrew and their children, Princess Beatrice and Princess Eugenie, have also been sent coveted invitations for a second, private evening reception for 250 being thrown by Prince Charles for the couple, Ferguson is off the guest list.
According to sources, the 58-year-old duchess has been complaining about the "snub" to anyone who will listen – even though many in the royal households think she should count herself lucky to be going at all.
Prince Harry personally intervened to make sure she had an invitation to his wedding to Meghan Markle and the first reception after years in the royal wilderness.
It is understood that Sarah has already been introduced to Markle by Harry, who is close to his cousins Beatrice and Eugenie, but it is thought that Charles, who is paying for the evening reception at Frogmore House, doesn't approve.
An insider said: "Numbers are limited to the evening party, she is not a member of the royal family any more and Prince Charles simply doesn't have time for her.
"He just can't see why she is still such a big part of his brother's life."
The duchess wasn't invited to the Duke and Duchess of Cambridge's 2011 wedding at Westminster Abbey, which was a much larger occasion than Harry and Meghan's "intimate" family ceremony, and took herself to a spa in Thailand to lick her wounds.
Although elations have thawed enough in recent years for Sarah to be invited to a small number of occasions, such as Royal Ascot and the family's summer sojourn at Balmoral, even the Queen has to take care that she doesn't appear when Prince Philip is around.
He is said to hold a deep-seated grudge against the duchess for the embarrassments he feels she has heaped upon his family over the years, once memorably describing her as "having no point".
He has never forgiven her for being photographed having her "toes sucked" by a pool in the South of France by her "financial adviser", John Bryan, in 1992, while still married to Andrew.
She has also tried Philip's patience with her profligate lifestyle, which has seen her come close to bankruptcy twice over the years, saved only by the generosity of her former husband.
Although Philip has not been seen in public for several weeks while he recovers at Windsor from a hip replacement, it is hoped he will be well enough to make an appearance on his grandson's big day, leaving many royal insiders speculating whether he will even acknowledge the duchess' presence.
One source said: "It will be fascinating to see if the Duke of Edinburgh acknowledges her.
"He has held a grudge against her for a long, long time and normally refuses to even be in the same room as her."
Earlier this year the Daily Mail told how Sarah had been invited to the wedding, contrary to reports that she had been banned, but the situation had caused a "massive headache" behind the scenes.
Senior royals, led by Charles and Philip, had made no bones about the fact that they didn't want the duchess there, but Harry, who is fond of her, held his ground.
He has kept in touch with his aunt over the years, and has previously holidayed at the Swiss ski chalet she bought for an estimated £13 million ($25.3m) a few years ago with her former husband.
"He absolutely, 100 per cent wants her there and has told palace officials that in no uncertain terms," a source said.
Despite their 1996 divorce, Sarah and Andrew remain extremely close and live at Royal Lodge, his Windsor home, together.