LONDON (AP) — Britain’s 99-year-old Prince Philip has spent a second night in a London hospital after being admitted earlier this week as a precaution due to feeling unwell.

Buckingham Palace said Wednesday that the husband of Queen Elizabeth II was admitted to the private King Edward VII Hospital on Tuesday evening. It said the admission was “a precautionary measure” taken on the advice of Philip’s doctor, and that the royal is expected to remain there for a few days of “observation and rest.”

His illness is not believed to be related to COVID-19. The queen and Philip 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, the duke has been staying at Windsor Castle, west of London, with the queen, 94, 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.