The Foreign, Commonwealth and Development Office has offered its “profound apologies” to the Danish royal family after inviting Crown Princess Mary to Queen Elizabeth’s funeral by mistake.
Official invites were sent to current heads of state, allowing each to bring one guest.
The Danish Royals confirmed Princess Mary's attendance on Sept13, writing: “HM The Queen and the Crown Prince Couple (will be) present at the State Funeral of Queen Elizabeth II.”
But Princess Mary, 50, was conspicuous by her absence at Westminster Abbey on Monday.
The Foreign Office on Tuesday admitted that a “regrettable error” had been made, which was blamed on the pressure of having to “send out many invitations within a short space of time”.
A spokesman said: “The FCDO has passed on their profound apologies to the Danish Royal Household through the Danish embassy.”
It came as the Foreign Office found itself at the heart of a mammoth last-minute operation, unprecedented in scale, with civil servants forced to coordinate the arrival in London of up to 500 heads of state and dignitaries.
More than 300 additional staff were seconded to the department, with the combination of security, protocol and logistics described as the most extensive piece of short-term planning it had faced since Winston Churchill's state funeral in 1965.
Aside from the practical elements of getting the foreign royals, prime ministers, presidents and governors-general to the capital, the Government also had to pull off the diplomatic masterstroke of keeping certain leaders at a suitable distance.
Ambassadors from North Korea, Nicaragua and Iran were invited, as well as South Korean president Yoon Suk-yeol and Isaac Herzog, the president of Israel, who recently suggested that Iran was leading the global “dark forces of hate”.
Current heads of state invited to the funeral were told they could bring one guest.
In the event, Queen Margrethe of Denmark, who ascended the throne on the death of her father, King Frederick IX, on January 14 1972, attended with her son, Crown Prince Frederik.
The pair were given front row seats in Westminster Abbey, directly opposite the King and Queen Consort.
Australian-born Princess Mary, who married Prince Frederik in 2004, met the late Queen on several occasions.
Questions over guests of other royal families
A Foreign Office source suggested that in the chaos of the moment, an error was made in suggesting that the guest of Queen Margrethe was also invited to bring a guest.
However, questions remain over why other royal families, including the Dutch, Spanish and Jordanians, had at least three guests each.
Denmark's Royal House told local news outlet BT: “There has been a regrettable error in the invitation from the British Foreign Office's protocol.
“It is thus only the Queen and the Crown Prince who, from the Danish side, will participate in Queen Elizabeth II's state funeral on Monday.”
Queen Margrethe, 82, enjoyed such a close relationship with the late Queen that she was one of few outside her immediate family who called her Lilibet.
She was among the first foreign royals to pay tribute to Her Majesty and struggled to hold back tears when she saw her coffin on Monday.
Both queens are great-great granddaughters of Queen Victoria and Margrethe said in a recent interview with ITV News that they shared great affection for one another.
“When I was growing up, I hoped I wouldn't be as young as that when my father died,” she said.
“It made an enormous impression on me. The fact that she was dedicating her life. I understood what that meant. This is for life. That is the whole point of my life. And I know she sees that too.”