The Queen has told guests at the state visit that her chest infection was a form of pneumonia.
Camilla is suffering from post-viral fatigue and is missing the outdoor elements of the Qatari visit after being urged by her doctors to take more time to recover.
It is understood Camilla’s condition was viral, not bacterial, and she no longer has pneumonia.
The Queen missed the outdoor ceremonial welcome on a chilly Horse Guards Parade on Tuesday.
Instead, she arrived at Buckingham Palace shortly beforehand, giving a smile as she stepped from her car at the Grand Entrance.
The Queen’s condition is not understood to be bacterial
Camilla, 77, waited for the Emir of Qatar and his wife inside, in the warm surroundings of the Grand Hall.
She contracted her illness after a long-haul trip to Australia and Samoa and has pulled out a series of engagements since then, including the Royal Variety Performance, the Gladiator II premiere and the Remembrance Day service at the Cenotaph.
It is understood the Queen was not taken to hospital with pneumonia, but is now facing bouts of extreme tiredness.
Her medical team has advised her to take each day as it comes.
She will join the King, the Prince and Princess of Wales, and the emir for a group photo before the banquet later. But she will take a short break before dinner while guests are met in a receiving line.
Kate makes rare appearance for Qatari visit
The royal visitors were officially welcomed in Whitehall by King Charles before the emir inspected a guard of honour by the 1st Battalion Welsh Guards on Horse Guards Parade.
The Prince and Princess of Wales had followed tradition and joined Sheikh Tamim bin Hamad Al Thani and the first of his three wives, Sheikha Jawaher, on their journey from their private residence to Whitehall.
The Prince and Princess of Wales at the ceremonial welcome for the emir of Qatar. Pic: PA
Pic: PA
The Princess of Wales greets dignitaries including Metropolitan Police Commissioner Mark Rowley. Pic: PA
The Princess of Wales greets Sheikha Jawaher, the wife of the emir of Qatar, in London. Pic: PA
Kate greets dignitaries as she arrives ahead of a Ceremonial Welcome for the Emir of Qatar
The royal women travelled in one chauffeur-driven limousine while the men travelled in another as they made their way to the parade ground where the waiting King greeted his guests warmly on a dais.
The princess looked relaxed as she stepped from a chauffeur-driven car with the prince, one of only a handful of official events Kate has carried out this year.
It’s one of the princess’s few official appearances during a year in which she has undergone treatment for cancer.
Kate has been gradually returning to royal duties after saying in September her chemotherapy treatment had finished, but she won’t go to the state banquet hosted by the King, where William will be a guest.
King Charles III and the emir of Qatar inspect troops on Horse Guards Parade. Pic: PA
Sheikh Tamim, 44, who was educated in Britain, has a busy schedule of engagements aimed at strengthening the ties between the UK and the Middle East nation, including meeting Prime Minister Sir Keir Starmer.