King Charles and Queen Camilla are set to lead the tributes at the Duchess of Kent's requiem mass at Westminster Cathedral.
According to Buckingham Palace, the monarch and his wife will be joined by other senior royals for the Catholic funeral service on Tuesday afternoon.
The duchess, who passed away on September 4 at her Kensington Palace residence, was the first Catholic Royal Family member in over 300 years and was the wife of the late Queen Elizabeth II's cousin, the Duke of Kent.
Tuesday's service will be historic, as it will be the first Catholic funeral for a Royal Family member in modern British history, as the duchess converted from the Anglican Church in 1944.
The Duke of Kent will attend Monday's private vigil alongside their "immediate family", which includes the couple's three children and ten grandchildren.
Their eldest son, George Windsor, Earl of St Andrews, 63, has three children: Edward Windsor, Lord Downpatrick, Lady Marina Windsor and Lady Amelia Windsor.
Lady Helen Taylor, 61, will attend with her four children, Columbus, Cassius, Eloise and Estella, whom she shares with her husband, Timothy Taylor.
The Kent's youngest son, Lord Nicholas Windsor, 55, has three sons: Albert, Leopold and Louis.
Moreover, the intimate gathering will include a rite of reception as the coffin arrives at Westminster Cathedral, followed by vespers for the dead.
Cardinal Vincent Nichols, the Cardinal Archbishop of Westminster, will lead the service with assistance from Bishop James Curry, Auxiliary Bishop of Westminster, and the Dean of Windsor.
The ceremony will take place at Westminster Cathedral, England's largest Roman Catholic church.
Members of The Royal Dragoon Guards will play a prominent role in the ceremonies, with a piper leading the hearse as it departs Kensington Palace on Monday.
The regiment, where the Duchess served as Deputy Colonel-in-Chief since its formation in 1992, will also provide the bearer party to receive her coffin at Westminster Cathedral.
Ahead of Tuesday's service, the coffin will remain overnight in the Chapel of the Blessed Virgin Mary.
After the mass concludes, the funeral car will transport the Duchess to her final resting place at the Royal Burial Ground at Frogmore, Windsor.