The Queen has cancelled a scheduled trip to Swansea tomorrow, suffering from gastroenteritis.
A Buckingham Palace spokesperson said: "She will be assessed in the coming days.
"Her Majesty is currently spending the weekend at Windsor, as usual".
Gastroenteritis is an infection of the stomach and large intestine, most commonly caused in this country by the norovirus and food poisoning, according to the NHS.
Queen Elizabeth was due to attend a military service in the south Wales city to celebrate St David's Day, but her duties will now be carried out by the Lord Lieutenant of West Glamorgan, Mr Byron Lewis.