Prince Philip, the Duke of Edinburgh died Friday at the age of 99, Queen Elizabeth announced.

“His Royal Highness passed away peacefully this morning at Windsor Castle,” the Royal Family wrote in a statement. “Further announcements will (be) made in due course. The Royal Family join with people around the world in mourning his loss.”

Philip spent a month in hospital earlier this year before being released on March 16 to return to Windsor Castle.

He had been transferred on March 1 to a specialized London heart hospital to undergo testing and observation for a pre-existing heart condition as he continued treatment for an unspecified infection. Philip had previously been admitted to the private King Edward VII Hospital hospital in February for several days after feeling unwell.

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

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

Philip, also known as the Duke of Edinburgh, retired from public duties in 2017 and rarely appears in public. During England’s current coronavirus lockdown, he has been staying at Windsor Castle, west of London, with the queen.

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 was named Augustus Philip Hawke Brooksbank, with one of his middle names a tribute to the Duke of Edinburgh.

The Associated Press contributed to this report.