The Queen is resting after suffering symptoms of gastroenteritis.
She was due to attend a military celebration in Wales during the country's St David's Day celebrations, but was forced to cancel her planned engagements because of the sickness.
Buckingham Palace declined to comment on her condition, but a spokeswoman earlier confirmed that the Queen would spend the weekend at Windsor.
She was expected to travel through Swansea to the Guildhall, where she would present St David's Day leeks to the 3rd Battalion The Royal Welsh, of which she is Colonel-in-Chief.
She was then due to attend a reception with members of the battalion and their families, before proceeding to the George Hall for lunch.
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.
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