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Queen Camilla has revealed to guests at Buckingham Palace that her chest infection was a form of pneumonia as she continues to suffer from the 'lingering' side effects.
The 77-year-old is suffering from post-viral fatigue and has missed the outdoor parts of the Qatari visit after being urged by her doctors to take more time to recover.
Camilla, who fell ill a month ago, is said to be feeling particularly 'fatigued' and pulled out of today's ceremonial welcome at Horse Guards Parade on doctor's advice.
Instead of attending the cold outdoor ceremony in London, the Queen was driven straight to the palace for a lunch to honour visiting head of state Sheikh Tamim bin Hamad Al Thani and the first of his three wives, Sheikha Jawaher.
She gave a smile as she stepped from her car at the Grand Entrance then waited for the Emir of Qatar and his wife inside, in the warm surroundings of the Grand Hall.
Once inside the palace, the Emir and his wife were introduced by Camilla and her husband King Charles III to the British suite - key members of the Royal Household - who had gathered to wait in a receiving line.
The King and Emir walked together, followed by Camilla and Sheikha Jawaher.
Camilla will later attend most elements of the State Banquet in the evening. It is understood her condition was viral, not bacterial, and she no longer has pneumonia.
Queen Camilla arrives at Buckingham Palace for the Qatari state visit to the UK today
Camilla is driven around the quadrangle to the Buckingham Palace Grand Entrance today
Queen Camilla arrives at Buckingham Palace during the Qatar state visit this afternoon
Sources stressed there was no cause for alarm, saying the chest infection had simply left the Queen 'with significantly diminished reserves of energy'.
As a result, they said, her doctors have advised some 'tweaks' to her programme.
Camilla's diagnosis was announced on November 5 when it emerged she had been forced to pull out of this year's Remembrance events.
The Queen contracted her chest infection 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.
A royal source said: 'Some days the Queen is on really good form. It's just one of those things.
'She has lost the coughing but the lingering side of it is bouts of extreme tiredness. There has to be a degree of flex in the Queen's diary at the moment.'
It comes as the Princess of Wales made a rare public appearance as the King officially welcomed the Emir of Qatar to the UK for a two-day state visit.
Charles greeted the Gulf state ruler in Whitehall as he began a busy round of public engagements aimed at strengthening the ties between the UK and the Middle East nation.
King Charles and Camilla view a display of Qatari items at Buckingham Palace today
Camilla was not present at Horse Guards Parade today for the ceremonial welcome
The Prince and Princess of Wales arrive at Buckingham Palace today during the state visit
The Prince and Princess of Wales in a carriage procession to Buckingham Palace today
King Charles III arrives at Buckingham Palace today with the Emir of Qatar Sheikh Tamim bin Hamad Al Thani and his wife Sheikha Jawaher, following the ceremonial welcome
Military pomp and pageantry was on display for the ceremonial welcome on Horse Guards Parade, where a guard of honour were formed by the 1st Battalion Welsh Guards stood waiting in two ranks to be inspected by the visiting head of state.
Prince William and Kate had followed tradition and accompanied the Emir and his wife from their private residence to Horse Guards Parade for the ceremonial welcome.
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.
After arriving at Buckingham Palace, gifts were exchanged, including a silver framed, signed photograph of Charles and Camilla to the Qataris.
The King also gave the Emir a hand-knotted Prayer mat, woven from Ghazni wool by Afghan women weavers from Bamiyan in central Afghanistan for his Turquoise Mountain charity.
The weavers involved in this piece are among the nearly 4,000 female weavers supported by Turquoise Mountain's pilot collaboration with the Qatar Fund for Development.
The Emir also gave the King a handmade Qur'an stand made by woodworker master Abdul Rahman Sha'aban from Damascus. Materials include mother-of-pearl, walnut, olive, maple, rose, cherry, eucalyptus and dyed walnut wood.
There was also a bespoke glove from the Ben Long Falconry, made in red deerskin from the Billy Tannery and bearing The King's cypher.
The Queen gave Sheikha Jawaher a silver and blue enamel round pill box, engraved with Her Majesty's cypher.
The royals view a display of Qatari items from the Royal Collection at Buckingham Palace today
The Emir of Qatar and King Charles III arrive to view the Qatari items at Buckingham Palace
The Prince and Princess of Wales view Qatari items at Buckingham Palace today
Charles also received a set of exquisite gifts from the Emir including a limestone and blue sodalite side table with geometric shell and brass inlays, inspired by the decoration of the Amiri Diwan building; a model of Al-Sanbook, a traditional sailboat used in pearl diving; a traditional Al-Sadu rug; a model of Oryxes, a symbol of Qatar, at Zekreet; a book documenting the history and details of the Amiri Diwan building in Qatar; and a photograph of Arabian horses.
Sheikha Jawaher gave the Queen a multi-stranded necklace of pearls.
The royal party - who were also joined by the Duke and Duchess of Edinburgh, the Duke and Duchess of Gloucester and the Duke of Kent - then enjoyed a private lunch.
Afterwards the King appointed The Emir as a Knight Grand Cross of the Order of the Bath, an ancient order of chivalry founded in 1725 by King George I.
The Emir presented the King with Qatar's highest national decoration, the Sword of the Founder Sheikh Jassim bin Mohammed bin Thani.
The group then made their way to the historic Buckingham Palace Picture Gallery where they examined a series of exhibits from the Royal Collection relating to Qatar, including a rare books from the royal library, a copy of Queen Elizabeth II's speech from the state visit of 2010, a silver model of a dhow and a gold mantle clock surmounted by a bronze horse presented in 2010 by the Emir, then heir apparent, to the-then Prince of Wales in 2010, emphasising the longevity of their friendship.
The King and Emir chatted animatedly, as did Camilla and the Sheikha. Camilla was heard to say: 'They'll explain to us about them', referring to the display items.
They were followed by William and Kate - who had exchanged her boots for burgundy Gianvito Rossi heels but kept her hat on - who spoke earnestly to each other, and then other members of the royal party.
The royals view a display of Qatari items from the Royal Collection at Buckingham Palace today
Sheikha Jawaher bint Hamad bin Suhaim Al-Thani and Camilla view items at Buckingham Palace
The Prince and Princess of Wales view Qatari items at Buckingham Palace today
Kate has been gradually returning to royal duties after she announced in September that she had completed her chemotherapy treatment and her focus was 'doing what I can to stay cancer free'.
However, Kate will not attend the state banquet hosted by the King in the palace's ballroom this evening, although William will be a guest.
On Horse Guards Parade the Emir was introduced to a long line of dignitaries, including Prime Minister Sir Keir Starmer, Foreign Secretary David Lammy and Home Secretary Yvette Cooper.
Military top brass were also present, including Admiral Sir Anthony Radakin, Chief of the Defence Staff, Admiral Sir Benjamin Key, First Sea Lord and Chief of the Naval Staff, General Sir Roland Walker, Chief of the General Staff and Air Chief Marshal Sir Richard Knighton, Chief of the Air Staff.
Charles escorted the Emir when he inspected the guard of honour walking a few paces behind his guest as he cast an eye over the Guardsmen.
In the hazy winter sunshine the Emir walked past the soldiers before returning to the dais with the King.
After the official welcome, crowds in The Mall were treated to a carriage procession towards Buckingham Palace with the King, the Emir and Sheikha Jawaher leading the way in the Irish state coach, followed by William and Kate and the Qatari prime minister, Sheikh Mohammed, in the Australian state coach.
The King and Qatar's Emir Sheikh Tamim bin Hamad al-Thani inspect a guard of honour
King Charles III shakes hands with Sir Keir Starmer at Horse Guards Parade today
As for Camilla, she pulled out of today's welcome at Horse Guards Parade on doctor's advice with the Palace saying she is still suffering from the 'lingering' side-effects of a 'nasty viral chest infection'.
The royal will attend most elements of the State Banquet this evening. So as not to exhaust her, she will greet their VIP guests and pose for photographs before retiring during a lengthy receiving line, but then join the dinner itself.
As for Camilla, sources described her as having had a 'nasty viral chest infection' which continues to 'have some lingering side effects, in particular leaving Her Majesty with significantly diminished reserves of energy.'
As a result, they said, her doctors have advised some 'tweaks' to her programme.
Her decision not to join the King at Horse Guards Parade for a full ceremonial welcome and open-air carriage drive back to Buckingham Palace on a cold morning was a sensible precaution given the nature of her illness, they stressed.
Her diagnosis was first announced almost a month ago on November 5 when it emerged that she had been forced to pull out of the Field of Remembrance at Westminster Abbey, followed by other events over the weekend, including the Cenotaph, to honour the nation's war dead.
As the daughter of a twice-decorated Second World War veteran, such duties are taken very seriously by the Queen - and her absence was a sign of how poorly she was feeling, sources close to her told the Mail.
Camilla has undertaken several public engagements since then, including the annual diplomatic reception, but has sometimes curtailed her time spent in public.
At a reception to honour the UK's film and television industry last month, the Queen discreetly coughed several times during her shortened appearance and gratefully sipped on a glass of water, saying she had felt 'absolutely rotten' last week but was definitely 'on the mend'.
Foreign Secretary David Lammy and Prime Minister Sir Keir Starmer at Horse Guards Parade
The King and Qatar's Emir Sheikh Tamim bin Hamad al-Thani inspect a guard of honour
Kate, Prince William and King Charles III at the welcome at Horse Guards Parade today
The Prince and Princess of Wales in a carriage procession to Buckingham Palace today
The Princess of Wales attends the ceremonial welcome at Horse Guards Parade today
Sources have stressed there has been 'no alteration' in any diagnosis or treatment being received, stressing Her Majesty just needs time to 'rest and recover fully, which is more challenging with a back-to back diary programme such as the state visit'.
No substantial changes to her other diary commitments are expected the rest of the week, though each day is being taken as it comes.
Guidance from her medical team suggests that in such cases post-viral fatigue symptoms can come and go and persist for several weeks.
At tonight's state banquet, Camilla will join photographs with the Emir and Sheikha Jawaher, but then retire briefly during the lengthy receiving line.
She will then attend the dinner as planned and join the King for the official farewells tomorrow.
Camilla has been praised for the way she has stepped up this year on behalf of her husband, after he was diagnosed with cancer at the start of the year and has continued to undergo treatment. Sources have described her as a 'trooper'.
Today's state visit is considered one of the highlights of the royal calendar and a show of royal 'soft diplomacy' in action.
Senior Qatari diplomats have told the Mail that they are 'deeply honoured' the King has chosen their country as the first Middle Eastern state to be invited for a full state visit to the UK since Charles' accession.
The country enjoys a good relationship with the UK diplomatically, with the Emir, 44, having studied in Britain - first at Sherborne and then Harrow - followed by a stint at the Royal Military Academy at Sandhurst.
There remains great admiration for the late Queen Elizabeth II, and Charles has met with the Emir on several occasions.
The Prince and Princess of Wales attend the ceremonial welcome at Horse Guards Parade
Life Guards (rear) and the Blues and Royals make their way to Horse Guards Parade today
King Charles III ahead of a ceremonial welcome for the Emir of Qatar in London today
The King and Qatar's Emir Sheikh Tamim bin Hamad al-Thani inspect a guard of honour
The Princess of Wales attends the ceremonial welcome at Horse Guards Parade today
Sheikha Jawaher bint Hamad bin Suhaim al-Thani at Horse Guards Parade in London today
The two countries also have strong trade, cultural and military links, and Qatar is a major investor in the UK, not just through property and construction projects, but in science and technology.
However concerns have been raised about the authoritarian nature of the Qatari ruling family and human rights, particularly as regards migrant workers.
The Emir, His Highness Sheikh Tamim bin Hamad Al Thani, will be bringing his first wife - and second cousin - Sheikha Jawaher bint Hamad Al Thani, with him on the visit.
She is the mother of four of his 13 children and normally accompanies him on official visits abroad. He also has two subsequent spouses.
The couple landed in the UK at London Stansted Airport yesterday where they were greeted on behalf of the sovereign by the Lord in Waiting, The Viscount Hood.
Today they were officially welcomed by William and Kate at their residence, before being escorted to Horse Guards Parade where the King waited for the ceremonial welcome.
National anthems were played and the Emir was invited to inspect a Guard of Honour.
The royal party then returned in a carriage procession to Buckingham Palace for a private lunch followed by an exhibition in the Picture Gallery of items from the Royal Collection Trust and Turquoise Mountain charity relating to Qatar.
William will later receive the Emir at Kensington Palace before the evening's state banquet, which he will attend but not his wife.
Tomorrow, 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.
Sheikh Tamim will also travel to Downing Street for bilateral talks with Prime Minister Sir Keir Starmer.
Kate has attended a number of high profile events such as Trooping the Colour, Remembrance Sunday and Wimbledon over the last 11 months since she effectively withdrew from public life to focus on her health.
But today's appearance was her most significant so far given that she will be playing a far more active role in events and be seen in public for a lengthy period of time.
Royal-watchers were delighted at the news that Kate is taking a further step forward as part of her staggered return to full-time royal duties.
Members of the King's Troop Royal Horse Artillery perform a gun salute in St James's Park
The King and Qatar's Emir Sheikh Tamim bin Hamad al-Thani inspect a guard of honour
Crowds gather by the Queen Victoria Memorial ahead of the ceremonial welcome today
Prince William and Kate attend the ceremonial welcome at Horse Guards Parade today
In September she spoke of her 'relief' at having finally completed her chemotherapy treatment, but stressed that the last nine months had been 'incredibly tough' for them as a family.
She was first admitted to hospital in January for what has only been described as major abdominal surgery.
However afterwards doctors found evidence that cancer had been present and recommended a course of preventative chemotherapy.
The couple chose to keep the news private for several weeks in order to come to terms with the news and protect their children, before the princess bravely went public with her diagnosis in March.
Talking of the 'complex, scary and unpredictable nature' of cancer journeys, Catherine said recently that doing what she could to 'stay cancer free is now my focus' and emphasised that her path to full recovery would be 'long'.
The Princess said she would undertake a few more public engagements in the coming months when she could but stressed that her focus would remain on her continued recovery and her young family.
