Buckingham Palace has said the Queen will not attend the register office wedding of her son Prince Charles and Camilla Parker Bowles on 8 April.
The Queen will attend the church blessing after the civil ceremony
BBC royal correspondents Nicholas Witchell and Peter Hunt outline the move.
Why has the Queen made this decision?
No one really knows, perhaps not even her officials, what considerations have figured in the Queen's mind to make this decision not to attend the wedding of such a close family member, said Nicholas Witchell.
We can say with some degree of confidence that the Queen has, in the past, not been the greatest fan of Mrs Parker Bowles, and that she's had reservations about the implications of such a marriage for the monarchy.
But she does take her role as Supreme Governor of the Church of England very seriously, and has said that while she lends her support to the rest of the process she feels it would be inappropriate to attend that particular part of the wedding.
There has been speculation that princes Edward and Andrew and the Princess Royal will also not attend the civil ceremony. Will they have been told not to go by their mother?
It is unlikely they will have been told by their mother, said Peter Hunt.
They are all pretty strong-willed and clear-cut individuals in terms of making up their own minds. It is not yet known whether they are definitely going.
This is not the wedding of Stephanie and Bob, aged 18, who suddenly wake up on morning and say 'let's get married'.
This is a wedding of a man who has had to grapple with this issue for years and years and years. He and his advisors have been involved in pretty detailed discussions about it for some weeks, if not months.
But Buckingham Palace is not yet in a position to say categorically whether his sister or brothers are coming - which leads one to suspect that they may not be going.
How will this latest twist in the tale of the pair's marriage be perceived?
People are likely to be surprised to hear about the Queen's decision, which she revealed nearly two weeks after the engagement was made public, said Nicholas Witchell.
Perception is crucial. Whatever the motivations of the Queen, it is how it will be interpreted that is both crucial and where the dangers lie, said Peter Hunt.
When you have people in this country that are saying 'I don't really care' it is a damaging development for the monarchy.
What the perception out there could be, is that this decision, however well-motivated and for whatever reason, would be interpreted as far from a ringing endorsement by the Queen of the wedding of her eldest son and her heir.
Will people assume this means the Queen is not entirely at ease with the marriage?
The royal palaces are insisting it's not a "snub" but some may find it difficult to believe that the Queen thought the decision would help the couple keep their wedding "low key", said Nicholas Witchell.
By staying away she will draw yet more worldwide attention, and could, problematically for the Royal Family, attract "worldwide ridicule".
We sometimes slip into the mistake of viewing them as we would view any families we know, said Peter Hunt.
They are not like us. They are royal. The monarchy is an institution that behaves in a certain way, and has had to behave in a certain way to survive and can be quite adaptable at times.
What does it say about the state of the wedding plans?
It shows they were not thought through very thoroughly in the first place, said Nicholas Witchell.
The plans have gone "from bad to worse" with the change of venue, and then arguments about the legality of the marriage.
It has not been the smooth and happy start to the wedding arrangements many involved would have hoped for.
The monarchy is an institution that prides itself on getting it just so and just right, said Peter Hunt.
That is what the Queen has survived on for decades.
Against that yardstick, this wedding and its preparations thus far do appear pretty shambolic.