Editor's note: Mark Saunders has been watching the British royal family for more than 20 years. He is author of "Diana and the Paparazzi" and "Prince Harry: The Biography."
London, England (CNN) -- For every wedding that's planned there's a snubbed relative. But none quite so publicly or brutally as Sarah Ferguson. In fact, not since the execution of Mary, Queen of Scots, has the British royal family shown such callousness towards a former member.
And what makes this particular snub so humiliating is that it didn't have to happen. Even though Fergie and Andrew, Duke of York have been divorced for more than 15 years, they are regularly seen in each other's company and insist they are still great friends.
Fergie's children, Princesses Beatrice and Eugenie, have been invited as, of course, has Prince Andrew. All of which makes Fergie's omission from the guest list seem so odd ...
In fact, it is the only real controversy of the wedding so far. After all, it would have been much easier to invite her. So, why didn't they? Here are five possible reasons:
1. Fergie's an embarrassment
Fergie has long been considered an embarrassment to the Royal Family; ever since she was photographed having her toes sucked by her financial adviser at a villa in Saint-Tropez, France. But last year came her biggest scandal to date, when she was secretly filmed attempting to sell "access" to her former husband for more than $700,000. The Royals are well aware that Fergie is broke and will do anything for money. Maybe a wedding invite was seen as too good an opportunity for Fergie to make some cash. A "be-my-date-at-the-royal-wedding" eBay auction would possibly cast a shadow over proceedings.
2. Prince William holds a grudge
Though Diana, Princess of Wales and Fergie were great friends, they fell out quite dramatically some six months before Diana died in 1997. Such was the bad feeling between the pair that Diana resolutely refused to take any of Fergie's calls. Is it possible that William himself, knowing the full extent of the rift, still bears a grudge?
3. There's simply no room
Buckingham Palace has always insisted the number of guests at the wedding is limited and that only true friends will be attending. However, as the guest list numbers 1,900 or more -- including, apparently, several kings from countries that no longer even have a monarchy -- surely space could have been found for poor Fergie. If you can invite 1,900 why not 1,901?
4. Andrew's bringing a date
Out of left-field this one, but a possibility; is Prince Andrew bringing a date? He has been seen in the company of several high-society beauties in the past few years and is a single man. Could the royals be trying to prevent any embarrassing scenes involving ex-wives and current girlfriends? Proceedings at the Abbey would be somewhat marred by a cat fight in the front aisle.
5. Fergie didn't want to go
Finally, has Fergie told William she doesn't want to go? We've already had a statement from her "office" claiming she has a commitment on April 29. Highly unlikely though, you really would have to be the coolest person on Earth to turn down this wedding.
While we'll likely never know the real reason, one can only assume the royal knives are still out for Fergie. Speaking to the BBC back in 1994, a royal aide described her as "vulgar, vulgar, VULGAR." It would appear Fergie has been consigned to the same dustbin as Wallis Simpson in Buckingham Palace's eyes.