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.
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.
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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 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.
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.
An amber warning has been issued for snow in a part of the UK, likely to cause travel delays and “stranding some vehicles and passengers”.
The warning, which covers south west England, also warns some rural communities could become cut off and is in place until 2pm this evening.
There is also a yellow weather warning for snow covering the southern counties of England until midnight on Wednesday.
The warning stretches from Kent to Cornwall and up to south London and the Met Office said between 2cm and 5cm of snow could accumulate fairly widely, with as much as 10cm over higher ground.
This week is expected to see the coldest nights of the year, with temperatures potentially reaching -14C on Wednesday night and -16C on Thursday night, both in the North East of England and Scotland, the Met Office said.
Weather warnings issued on Tuesday for snow and ice covering the parts of Wales, the North West of England, west and northern parts of Scotland as well as Northern Ireland will remain in place until midnight tomorrow.
The forecaster said some roads and railways are likely to be affected and there could be icy patches on untreated roads.
Meanwhile the Environment Agency has said at least 300 properties have flooded across England since New Year’s Eve. It estimates more than 41,000 properties have been protected.
Heavy rainfall over the New Year caused significant river and surface water flooding across the North West of England and Yorkshire and snowmelt has brought further disruption to parts of England, particularly the Midlands, the agency said.
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Man says flooding ‘came out of nowhere’
Floods minister Emma Hardy said: “My sympathies go out to the people, businesses and communities impacted by the recent flooding across the country.
“I want to express my heartfelt thanks for the vital work that the Environment Agency and emergency services are doing to keep people safe. People must continue to follow their advice and sign up for flood warnings.”
Flood warnings
Some 100 flood warnings were in force across England on Wednesday, with people urged to remain vigilant over the coming days.
A danger-to-life warning was issued on Tuesday morning for the River Soar near Barrow upon Soar, Leicestershire, but was later removed.
People living in caravan parks in the area were urged by the Environment Agency to act, with a large-scale evacuation needed to save lives.
Firefighters have rescued dozens of people across Leicestershire since Monday, Leicestershire Fire and Rescue Service said.
Hundreds of schools were closed across the UK, with road and rail links blocked, as Manchester, Bristol and Liverpool John Lennon airports suspended flights because of the conditions.
UK Athletics Ltd and a former head of sport for a major London games have been charged with manslaughter following the death of Paralympian Abdullah Hayayei, the Crown Prosecution Service said.
United Arab Emirates athlete Mr Hayayei died aged 36 after a metal throwing cage fell on him while he was shot put training at Newham Leisure Centre in London in 2017.
The Paralympian was pronounced dead at the scene after he was struck by a metal pole in the incident.
He was training in preparation to represent his country in the shot put, discus and javelin at the World Para Athletics Championships which began in London later that week.
Teams and coaches from several nations were present at the time of the tragedy.
Keith Davies, 77, the former head of sport for the 2017 World Para Athletics Championships, has been charged with gross negligence manslaughter and a health and safety offence.
UK Athletics has been charged with corporate manslaughter and a Health and Safety at Work Act offence.
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Both UK Athletics and Davies are due to appear at Westminster Magistrates’ Court on Friday 31 January.
Malcolm McHaffie, head of the CPS Special Crime Division, said: “The Crown Prosecution Service reminds everyone that criminal proceedings are active and both UK Athletics Ltd and Davies have the right to a fair trial.
“It is extremely important that there be no reporting, commentary or sharing of information online which could in any way prejudice these proceedings.”
The Met Police launched an investigation following Mr Hayayei’s death on 11 July 2017, with the Crown Prosecution Service saying today that it has authorised charges following a review of evidence provided by the force.
Mr Hayayei made his Paralympic debut in Rio de Janeiro in 2016 and was set to feature in his second World Championships in 2017.
Ed Warner, who was co-chair of London 2017, said after Mr Hayayei’s death: “This is a devastating event and everyone within the London 2017 Organising Committee is shocked and saddened.
“We will be working closely with all the competitors and teams over the days ahead to offer support wherever it is needed.”
The 14-year-old who was stabbed to death on a bus in southeast London yesterday has been named as Kelyan Bokassa.
He was stabbed to death on a bus in Woolwich Church Street, close to the Woolwich Ferry, just before 2.30pm on Tuesday.
The teenager died at the scene shortly after medical help arrived.
No arrests have been made so far but officers are appealing for witnesses who were on the 472 bus.
Police called the stabbing a “horrific attack”.
Detective Chief Inspector Martin Thorpe, who is leading the investigation, said: “Following Kelyan’s tragic death yesterday, specialist officers are now leading the investigation which is progressing at pace.
“If anyone saw this horrific attack or was in the area then I urge them to come forward and speak to officers.
“We’re particularly interested in any mobile phone footage that captured this incident or any dash cam or doorbell footage from around the surrounding area at the time of this incident.”
This breaking news story is being updated and more details will be published shortly.