The Queen Consort has been nursing a broken toe for over a fortnight, according to reports.
The injury occurred before the death of Queen Elizabeth II last Thursday, it is understood.
Camilla has accompanied the King to several services and events following his ascension.
The 75-year-old's injury is not believed to have required hospital treatment.
On Wednesday, she was driven with the Prince of Wales to a service at Westminster Hall ahead of the Queen lying in state.
However, the injury is not thought to be behind her not joining the 30-minute family procession led by the King.
She was on her feet throughout Wednesday's service, which preceded Westminster Hall being opened for members of the public wishing to pay their respect to the UK's longest-serving monarch.
The Sun reported an insider as saying: “Camilla has broken her toe and although it's healing it is very, very sore.
“It is terrible timing but she is soldiering on and not letting a broken toe get in the way of duty and doing what she needs to do for the King, the family and the country.”
A spokesman for the Queen Consort said that they did not comment on medical matters.
It was reported earlier this week that Camilla has been tipped to continue one of the former monarch's favourite hobbies.
Plans for her to take over the Queen's racehorses are likely to be confirmed in the coming weeks, according to reports.
Camilla and King Charles III have had a number of their own thoroughbreds in training over the years, competing at tracks including Cheltenham and Ascot.
But the Queen has always headed the royal family's interest in what is known as the 'sport of kings.'
Sources say her daughter-in-law Camilla is expected to take the reins as racing mourns the loss of its biggest supporter.
A senior racing source told the Mail on Sunday : “I think now it will be Camilla – she will be the figurehead. The plan has always been that Charles and Camilla would take on the legacy.
“Camilla obviously adores horses and is hugely passionate about racing. She really loves it, particularly Cheltenham – the jumps. I think there’s a pretty clear plan, given the Queen’s passion and love for racehorses.”
