"He has been displaying mild symptoms, but otherwise remains in good health and has been working from home throughout the last few days as usual," a statement released on March 25 say.
Duchess Camilla was also tested and was found negative.
"In accordance with government and medical advice, the Prince and the Duchess are now self-isolating at home in Scotland," the statement continues. "The tests were carried out by the NHS in Aberdeenshire where they met the criteria required for testing."
The Duke and Duchess of Cornwall are understood to be isolating separately at Birkhall, their home on the Balmoral e
"It is not possible to ascertain from whom the Prince caught the virus, owing to the high number of engagements he carried out in his public role during recent weeks."
According to HELLO! UK, doctors think Charles became contagious on March 13.
Buckingham Palace has also released a statement, saying the Queen remains in good health.
"The Queen last saw the Prince of Wales briefly after the investiture on the morning of 12th March and is following all the appropriate advice with regard to her welfare," the statement reads.
According to the Court Circular, on that day, the 71-year-old carried out an investiture on the Queen's behalf at the Palace. That evening, he also attended a dinner for Australian bushfire relief. On March 11, he stepped out to the London Palladium for the Prince's Trust Awards. He went to the WaterAid Summit on March 10, and attended Commonwealth Day services at Westminster Abbey with the rest of the Royal Family on March 9.
At his most recent engagements, Charles had been placing his palms together and greeting those he met by saying "namaste" instead of shaking hands.
The Duke of Cornwall is not the first royal to test positive for COVID-19, but he is the first member of the British Royal Family to have the illness.
Last week, Karl von Habsburg, the ancestral Archduke of Austria and Prince Albert of Monaco both revealed they had tested positive for coronavirus. Both said they were following medical advice and had only mild symptoms so far.