A palace spokeswoman said: "The Queen will no longer visit Swansea as she is experiencing symptoms of gastroenteritis. She will be assessed in the coming days. Her Majesty is currently spending the weekend at Windsor, as usual."
The Queen and the Duke of Edinburgh are due to fly out to Rome for a two-day visit next week, which could now be in doubt. A palace spokesman said the visit was still due to take place, but a decision would be made after she was assessed.
The Queen, who is 86, has undertaken a number of engagements during the last week. On Tuesday she met the Archbishop of Canterbury at Buckingham Palace, and on Thursday she presented a host of Olympic stars with honours during an investiture ceremony.
Swansea Council said the ceremony would go ahead as planned with the Lord Lieutenant of West Glamorgan, Byron Lewis.
The Queen has rarely missed a royal engagement due to ill health in recent years. She was last forced to pull out of an investiture ceremony in October last year after suffering with a bad back. She also missed a visit to the British Museum in October 2011 because of a cold.