The Duchess of Cornwall has pulled out of a memorial event due to a chest infection, Clarence House has said.
The 72-year-old was due to visit the Field of Remembrance in the grounds of Westminster Abbey with the Duke and Duchess of Sussex on Thursday.
However, a spokeswoman said that Camilla's doctor told her to cancel her engagements because the infection had "got progressively worse" of late.
The duchess also missed an event on Wednesday due to the illness.
She and the Prince of Wales were pictured on a visit to a farmers' market in west London earlier that day.
But the duchess was forced to cancel her scheduled appearance at a gala dinner in London to mark the 200-year anniversary of Prince Albert's birth.
Members of the Royal Family have been taking part in memorial events ahead of Remembrance Sunday.
On Thursday, Camilla, who is patron of the Poppy Factory, had been due to attend the 91st Westminster remembrance alongside Prince Harry and Meghan.
It is thought the event would have been her first public outing alongside the couple without her husband, the Prince of Wales.
The Duke and Duchess of Sussex paid their respects to servicemen and women who have died in conflicts by planting tiny crosses in the grounds of Westminster Abbey.
About 70.000 small crosses have been erected by regiments and military units since the tradition began in 1928.
During the visit, Harry spoke with his great-grandmother's former driver Arthur Barty, who was representing his former unit The Black Watch.
Mr Barty, who drove the Queen Mother for 27 years until her death in 2002, said: "I covered almost 100,000 miles with the Queen Mother.
"I never thought for a minute I would meet His Royal Highness or Her Royal Highness but it was an absolute pleasure to chat to them."
On Sunday, the Queen will lead senior royals in paying tribute to those who have lost their lives in armed conflict during the Remembrance Sunday service at the Cenotaph in Whitehall.
Last year, the Duke of Edinburgh, 97, did not attend the service, having retired from his public duties in 2017.