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LONDON, ENGLAND – JUNE 12: (L-R) Queen Elizabeth II and Prince Philip, Duke of Edinburgh during “The Patron’s Lunch” celebrations for The Queen’s 90th birthday at The Mall on June 12, 2016 in London, England. (Photo by Jeff Spicer/Getty Images)
Queen Elizabeth II sits with Prince Philip, Duke of Edinburgh as she delivers her speech during the State Opening of Parliament in the House of Lords at the Palace of Westminster on June 4, 2014 in London, England. (Photo by Ray Collins - WPA Pool/Getty Images)
Queen Elizabeth II sits with Prince Philip, Duke of Edinburgh as she delivers her speech during the State Opening of Parliament in the House of Lords at the Palace of Westminster on June 4, 2014 in London, England. (Photo by Ray Collins – WPA Pool/Getty Images)
Undated portrait of Prince Philip of Greece, later Philip Mountbatten, Duke of Edinburgh. (Photo by -/AFP via Getty Images)
Queen Elizabeth II poses with her husband Prince Philip, Duke of Edinburgh and their children Prince Charles of Wales (L) and Princess Anne of England (R) in October 1950 in London. (Photo by – / AFP)
Undated picture showing Prince Charles of Wales with his father Prince Philip of Edinburgh on board a boat. (Photo by -/AFP via Getty Images)

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.