London (CNN)Britain's Prince Philip is likely to remain in the hospital for observation and rest over the weekend and into next week, a royal source told CNN on Friday.
Philip, the husband of Queen Elizabeth II, was admitted to a London hospital on Tuesday evening after "feeling unwell," Buckingham Palace announced earlier this week.
"As we have said previously, the doctor is acting with an abundance of caution. The Duke remains in good spirits," the source said on Friday.
The precise reason for the admission has not been disclosed.
The 99-year-old stepped back from public life in 2017 and has been taken to the hospital several times in recent years. In December 2019, he received hospital treatment for a pre-existing condition.
According to a royal source, his latest admission was not an emergency and the Duke traveled to King Edward VII hospital by car and walked in unaided. The source added that the illness was not Covid-19 related.
The Queen has remained at Windsor Castle, to the west of London, the source said at the time.
Prince Philip and Queen Elizabeth have both received at least one dose of a Covid-19 vaccine.
Earlier on Friday, Buckingham Palace announced that Prince Harry and Meghan, Duchess of Sussex would not be returning as working members of the royal family.