The Queen is to miss the ceremonial welcome during the Qatari state visit after suffering significantly diminished reserves of energy following her chest infection.
Doctors have urged Camilla, 77, to take time to rest and recover fully because of the lingering effects of the illness, which she contracted a month ago following her long-haul trip to Australia and Samoa.
The King and the Prince and Princess of Wales are set to welcome the Emir of Qatar and his wife to the UK on Tuesday during the traditional formal welcome outdoors on Horse Guards Parade.
The princess will meet Sheikh Tamim bin Hamad Al Thani and his wife, Sheikha Jawaher bint Hamad bin Suhaim Al Thani, with William in London, and accompany them to the parade ground to be greeted by the waiting King and senior government figures.
Charles, Kate, William and their guests will afterwards take part in a carriage ride back to Buckingham Palace, attend a lunch and visit a Picture Gallery exhibition in the royal residence.
But she will join the Emir and his wife for the lunch and was hoping to be able to view the display in the gallery.
Camilla will still be present at the glittering state banquet in the evening and will pose for a group photo at the start.
But she will take a short break before dinner while guests are being met in a receiving line.
Camilla has pulled out of a number of engagements over the past few weeks including the Royal Variety Performance, the Gladiator II premiere and the Remembrance Day service at the Cenotaph.
With temperatures expected to hover around 7C in London tomorrow, it is likely royal doctors were keen for Camilla to miss the ceremonial welcome to avoid her standing in the chill December air for an extended period of time.
The ceremony, which sees the royal party gather on a royal dais, involves the King inspecting a Guard of Honour with his visiting head of state.
No major changes are expected to her diary for the rest of the week, but the Queen has been advised by her medical team to take each day as it comes.
Meanwhile, the occasion marks the start of a busier week for Kate, who is slowly increasing her public appearances after finishing her chemotherapy treatment in the summer.
The princess will be hosting her annual Christmas carol service at Westminster Abbey on Friday, when she is expected to be joined by her children and husband for the event.
He set up Qatar Sports Investments in 2005, which owns the French football team Paris Saint-Germain.
On Wednesday, the final day of the two-day visit, the Emir will visit Sandhurst before travelling to the Palace to bid farewell formally to the King and Queen, with Camilla expected to be there.
Sheikh Tamim will also travel to Downing Street for bilateral talks with Sir Keir Starmer.
But the country has faced criticism over its human rights record, in particular its criminalisation of same-sex relationships.
