Queen Camilla has pulled out of attending the funeral of Katharine with "sincere apologies", with the royal household saying she is battling acute sinusitis.
Yet the palace says the Queen, in spite of this uncomfortable inflammation of the sinuses, still plans to take part in the official trip of President Trump, who will be in Windsor on midweek.
The King and additional senior members of the royal family are expected later at the funeral of Katharine at the historic cathedral.
It is the first Catholic funeral for a member of the Royal Family in recent history – and a blessing from the Pontiff will be presented during the service.
The monarch's ailment was disclosed by the royal communications team prior to the funeral, which will occur on Tuesday afternoon.
"With great regret, Queen Camilla has withdrawn from attendance at this day's Requiem Mass for the Duchess Katharine as she is managing acute sinusitis. She sends her condolences will be with the Duke and all the family," stated the statement.
The casket of the royal was brought to the house of worship on Monday, covered in the flag and with floral arrangements among the flowers, in honour of the duchess' origins.
The duchess was the oldest member of the monarchy when she died earlier this month at the age of 92. She was wed to the close relative of the late Queen Elizabeth II.
There have been many tributes of her kindness and her dedication to charities.
Cardinal Vincent Nichols, who will officiate the ceremony, has mentioned how the duchess often offered her time at the Passage charity for the homeless, which is based nearby to the church.
Reflecting ahead of the service, he described her as a "approachable individual" who had assisted in the dining area and with cleaning at the organization.
"So as well as combining official responsibilities, she was deeply involved in helping others," Cardinal Nichols told.
This is a historic occasion in terms of being a Catholic service for royalty, but the Cardinal said it should be seen primarily as a "family funeral where the members of the family gather round."
"A beloved elder has died. A lady who was deeply cherished within that family and also, evidently, by much of the public," stated Cardinal Nichols.
The late royal had a deep passion of music and had served as a primary school educator, calling herself Mrs Kent, with the pupils not knowing anything about her background.
Music at the service will feature Ave Verum Corpus by the composer, which was selected by the duchess as her favourite piece when she was a participant on the a popular radio program in 1990.
A piper will perform the lament Sleep, Dearie, Sleep, which was played at the memorial of the late Queen, which was three years ago this time.
There will also be performances by JS Bach and from the requiem by French musician Maurice Durufle.
The duchess will also be remembered as a familiar figure at the tennis tournament, where she handed over trophies – and consoled those who had lost, including a tearful a tennis professional in 1993.
After the funeral at the cathedral, the remains will be transferred to the royal burial ground in the estate, on the royal grounds.
