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.

Camilla, 77, 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 the King and Queen 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.

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 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.”