The 95-year-old was with Prince Charles this week.
Queen Elizabeth is reportedly being monitored closely after Prince Charles tested positive for Covid-19.
It is believed the Prince of Wales was with the Queen just two days before his test came back positive.
This is the second time Prince Charles has been diagnosed with Covid-19.
It is understood the prince was taking a routine test. He is not currently symptomatic, sources say.
Clarence House confirmed:
"This morning The Prince of Wales has tested positive for COVID-19 and is now self-isolating.
"HRH is deeply disappointed not to be able to attend today's events in Winchester and will look to reschedule his visit as soon as possible."
The Queen's medical team is monitoring her health closely. However, she is not displaying any Covid symptoms as of yet.
Prince Charles will have to isolate for the next 10 days, but he can leave isolation if his test is negative on days 5 and 6.
A source close to the royals said the Palace won't be sharing details about the Queen's health.
The Duchess of Cornwall is not testing positive for Covid-19.
She has continued with her royal engagements.
The Queen, Prince Charles, and Camilla are all fully vaccinated. They have also received their booster shots.
The Prime Minister issued a statement following the news. A spokesperson stated: "The Prime Minister would like to wish the Prince of Wales a speedy recovery."
Prince Charles also tested positive for Covid-19 in 2020. The Prince of Wales said he was "lucky" because his symptoms were only mild. However, his son Prince William tested positive but was hit "pretty badly" by Covid.
