Since he retired in 2017, the duke has largely lived at Wood Farm on the Queen's Sandringham Estate, apart from brief visits to Windsor and a longer summer stay at Balmoral.
His birthday will be marked by lunch with the Queen, as well as Zoom and telephone calls with family and friends – including Prince Charles in Scotland, the Duke and Duchess of Cambridge in Norfolk and Harry and Meghan in Los Angeles.
A small number of public birthday wishes will be released on social media by family members, while Buckingham Palace hopes to mark the occasion publicly in some small way, "depending on how His Royal Highness feels".
But the mantra of the day is very much "low key".
"He's not one for fuss, as you know," a source said.
Philip's cousin and childhood friend Myra Butter confirmed that he has always insisted on "no fuss, no bother".
Lady Butter, 95 – who shares a great-great-grandfather, Tsar Nicholas I of Russia, with Philip – said: "He has such an incredibly active mind. And he's a great reader, so interested in things.
"He's got enormous knowledge and I am absolutely certain there must be a link between that and his long and healthy life. He's such an enigma, really.
The Duke has suffered several bouts of poor health in recent years and was in hospital just before Christmas but is said to be far better now, despite feeling the effects of his advancing years.
Lady Butter said: "The secret is that he just does everything he has previously done, but slower.
"He still enjoys the greatest role of all, supporting the Queen. As for a party to celebrate turning 99? Goodness me, no! He never wants a fuss, ever!"
