The Queen has been admitted to a London hospital after being diagnosed with gastroenteritis.
All official engagements for this week will be either postponed or cancelled as a precaution, including the Queen’s planned visit to Italy, Buckingham Palace said.
The visit is taken as a precautionary measure as doctors say they will be able to better assess the Queen’s condition in a hospital, the Palace added.
The Queen, who is 86, and the Duke of Edinburgh were scheduled to visit Rome from Wednesday to Thursday.
She is expected to spend two days in King Edward Hospital in Marylebone, central London, but Buckingham Palace said today her condition is not deteriorating.
The monarch was due to attend a military celebration in Swansea on Friday as part of the city’s St David’s Day celebrations.
The Queen, 86, was due to spend three hours a meeting soldiers and their families before a lunch at the George Hall.
She had also planned to present St David’s Day leeks to members of 3rd Battalion The Royal Welsh Regiment.
The ceremony continued in her absence, but was carried out by the Lord Lieutenant of West Glamorgan, Byron Lewis.
The Queen has rarely taken time off due to illness during her six decades as monarch.
Queen Elizabeth was last admitted to hospital in 2009 for a routine operation to her knee to remove a torn cartilage.
In 1994 she was also hospitalised when she broke her wrist in a riding accident and in 1982 when she had a wisdom tooth removed.
In recent years she has otherwise been in robust health and is known not to make a fuss in times of sickness or discomfort.
She has, however, suffered occasional problems with her back. In October, she was forced to cancel an appearance at a Windsor Castle investiture ceremony, and also had to miss a church service in Scotland a few days earlier.
In 2006, she suffered a strain to her back during her annual summer break at Balmoral which led to her cancelling a tour of Arsenal’s new stadium.
She also missed a visit to the British Museum in October 2011 because of a bad cold.
Disappointed locals took to Twitter to voice their upset at the change of plans for Swansea. Becca Masters commented ‘poor Queenie’ while Peter Grant added ‘Get well soon your Majesty!’
June Marcroft added: ‘Jeez, she’s human after all, I thought she was bionic.’
The Queen’s most recent appearance was to present a host of Olympic stars with honours during an investiture ceremony at Buckingham Palace on Thursday.
Gastroenteritis is an infection of the stomach and bowel, with common symptoms including vomiting, diarrhoea and dehydration.
In England, the two most common causes of gastroenteritis in adults are the norovirus and food poisoning, according to NHS Choices.
In June, the Duke of Edinburgh succumbed to a bladder infection thought to be brought on after being forced to stand for several hours in cold, wet and windy conditions on a barge sailing down the Thames during the Jubilee River Pageant.
The 91-year-old was taken into hospital just hours before the Queen’s Diamond Jubilee’s closing celebrations at Buckingham Palace, where she was joined by some 12,000 people.
The illness forced him to miss several other key events, including a service of thanksgiving at St Paul’s Cathedral.
Read more: DailyMail