Prince Philip, Queen Elizabeth II’s husband of 71 years, marked his 98th birthday on Monday.
Mr Philip, whose official title is the Duke of Edinburgh, was expected to celebrate in private, with no public events scheduled.
He is the oldest and the longest-serving consort in British history, described by the 93-year-old queen as her “constant strength and guide.”
His sons Prince Charles and Andrew, the Duke of York, wished him a happy birthday via Twitter as did his grandson Prince William and his wife Kate, the Duchess of Cambridge.
Mr Philip is known for combining a strong sense of duty with public popularity and a propensity for gaffes.
He has attended few engagements with the queen since announcing his retirement from most public duties in May 2017.
Philip did not join the rest of the royal family at Saturday’s annual Trooping of the Colour parade to mark the queen’s official 93rd birthday.
He was not seen during last week’s royal banquet and other events to mark a state visit to Britain by U.S. President Donald Trump and his family.
However, he was photographed in May at a wedding and in charge of a carriage at a royal horse show.
Prince Philip also appeared in a photograph in May with his latest great-grandchild Archie, the infant son of Prince Harry and Meghan, the Duchess of Sussex.
The queen and Philip have four children, eight grandchildren, and eight great-grandchildren.