LONDON (AP) — Prince Charles went to a London hospital on Saturday to visit his father, Prince Philip, who was admitted earlier his week for “observation and rest” after falling ill.

Charles arrived at the private King Edward VII’s Hospital by car in the afternoon and stayed for about half an hour. The hospital’s website says visits are only allowed in “exceptional circumstances” because of the coronavirus pandemic.

Philip, 99, was admitted to the hospital on Tuesday on the advice of his doctor in what Buckingham Palace described as “a precautionary measure.”

The husband of Queen Elizabeth II is expected to remain through the weekend and into next week.

Philip’s illness is not believed to be related to COVID-19. Both he and the queen, 94, received a first dose of a vaccine against the coronavirus in early January.

Philip, who retired from public duties in 2017, rarely appears in public. His most recent public event was a military ceremony at Buckingham Palace in July.

During England’s current coronavirus lockdown, Philip, also known as the Duke of Edinburgh, has been staying at Windsor Castle, west of London, with the queen, who has performed duties such as meetings with dignitaries remotely.

The royal household is planning celebrations to mark Philip’s 100th birthday on June 10, lockdown restrictions permitting.

Philip married the then-Princess Elizabeth in 1947 and is the longest-serving royal consort in British history. He and the queen have four children, eight grandchildren and nine great-grandchildren.

The youngest great-grandchild, son of Princess Eugenie and her husband Jack Brooksbank, was born Feb. 9 and has been named Augustus Philip Hawke Brooksbank, with one of his middle names a tribute to the Duke of Edinburgh.