The Queen has cancelled an engagement in Swansea as she is suffering from the symptoms of gastroenteritis.
Buckingham Palace confirmed the Queen is unwell and will "be assessed in the coming days".
The 86-year-old monarch was due to visit Wales today to present St David's Day leeks to 3rd Battalion The Royal Welsh.
She is currently at Windsor Castle, where she is spending the weekend "as usual", the palace said in a statement.
The Queen is due to travel to Rome on March 6 to visit Italy's President Napolitano.
She last had to cancel an event - an investiture ceremony at Windsor Castle - due to chronic back pain in October 2012.
She also missed a church service at Sandringham two days before Christmas last year because she was "getting over the tail end of a cold".
The Queen presented Olympic stars, including Jessica Ennis, Nicola Adams and Greg Rutherford, with honours at Buckingham Palace on Thursday.
A palace spokesman said the Queen's visit to Rome was still due to take place but a decision would be made after further assessment of her condition.
Meanwhile, Swansea Council said the ceremony would go ahead today as planned with the Lord Lieutenant of West Glamorgan, Byron Lewis.
A council spokesman said: "Due to minor illness, Her Majesty The Queen is unable to attend Saturday's event.
"The ceremony will continue as planned and will be carried out by the Lord Lieutenant of West Glamorgan, Mr Byron Lewis.
"The Lord Lieutenant will be met upon arrival at the Guildhall by Brigadier Philip Napier, Colonel of the Regiment.
"The parade will present arms for a royal salute and play the national anthem before the Lord Lieutenant is invited to present leeks to a representative body of the regiment.
"Following the parade, the Lord Lieutenant will attend a reception with members of the battalion and their families, before proceeding to the George Hall for lunch.
"The event will take place in and around the Brangwyn Hall and public viewing areas will be available."
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.
The Queen has rarely missed events during her 61-year reign, even though she carries out more than 300 engagements a year.