The King has become the first British monarch in four centuries to attend a Catholic funeral, after joining members of the royal family for a requiem mass for the Duchess of Kent.

The duchess, Katharine Worsley, joined the royal family when she married Prince Edward, the Duke of Kent and cousin of the late Queen Elizabeth in 1961.

She 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, the duchess was also known for converting to Catholicism in 1994 – 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 the current King Charles, who is Supreme Governor of the Protestant Church of England, to attend Tuesday’s funeral for the duchess at London’s Westminster Cathedral.

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. The Queen was a last-minute withdrawal from the service as she is recovering from acute sinusitis.

The King’s only sister Princess Anne was also there, with her husband Sir Timothy Laurence. His younger brother Prince Andrew and his ex-wife Sarah were among the crowd, along with the Duchess of Edinburgh. The King’s youngest brother, Prince Edward, is in Papua New Guinea for the 50th anniversary of its independence.

“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 had 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 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 later Wednesday (Australian time).

The King has for many years stated that he wishes to protect all faiths, and he and the Queen 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 AAP