The 99-year-old was admitted to the private King Edward VII hospital in Marylebone, London, on Tuesday night as "a precautionary measure" after he reported feeling unwell.
The Queen's husband is said to have been well enough to walk into the facility unaided, however, and although the nature of his illness is unknown, it is not thought to be related to coronavirus.
Charles, Philip's oldest son and the first in line for the throne, paid a visit to the hospital on Saturday afternoon and stayed for around 30 minutes.
Under current coronavirus guidelines, hospital visits are only permitted in exceptional circumstances - although it is unclear whether Charles's visit was permitted due to his Royal status, Philip's age or the health circumstances involved.
Philip - the longest-lived male member of the Royal Family in history - retired from official duties in 2017 at the age of 96.