The duchess, Katharine Worsley, who joined the royal family when she married Prince Edward, the Duke of Kent, in 1961, died on September 4, aged 92.
Best remembered for her close association with the Wimbledon tennis tournament, where from 1969 she helped to hand out the trophies, she was also known for converting to Roman Catholicism in 1994, becoming the first British royal to do so since King Charles II, who turned to the faith on his deathbed in 1685.
Thus it was a break in tradition for Charles, the Supreme Governor of the Protestant Church of England, to attend the funeral for the duchess at London's Westminster Cathedral, where the service was conducted by Cardinal Vincent Nichols, the leader of Catholics in England and Wales.
Also present were the King's son and heir Prince William and his wife Kate, although Charles' wife Queen Camilla had to pull out as she was recovering from acute sinusitis.
Princess Anne and her husband Timothy Laurence also attended, as did Prince Andrew and his ex-wife Sarah.
"The funeral will be of great historical significance," Catherine Pepinster, former editor of the Catholic weekly review The Tablet, told the Sunday Times.
"This is a huge step forward in ecumenical relations."
The duchess preferred to be known as Mrs Kent and dropped her HRH style, retreating from royal life to spend more than a decade secretly teaching music in a state primary school in Hull.
Following the funeral, the duchess' coffin was to be taken to a burial ground at Frogmore on the royal estate surrounding Windsor Castle, which is preparing to host US President Donald Trump for a state visit from Wednesday.
Charles has for many years stated that he wishes to protect all faiths, and he and his wife Camilla were among the last official visitors to see Pope Francis before his death in April.
Pope Leo XIV said he was saddened to learn of the duchess' death in a message to the King which was read out at the service.
with PA
