Philip, the Duke of Edinburgh, is spending the day privately at Windsor Castle, west of London, where he and the 94-year-old monarch have been staying during Britain's coronavirus lockdown.
That means the rest of the royal family will have to send their best wishes to him by video calls.
"I haven't seen my father for a long time. He's going to be 99 next week," heir-to-the-throne Prince Charles and the oldest of his four children told Sky News last week.
"Facetime is all very well ... You really want to give people a hug."
The royals make little fuss about their birthdays in public but this year celebrations have been even more muted.
Traditionally, the prince's birthday would be celebrated with gun salutes in London but with social gatherings banned, the queen felt it would be inappropriate for such royal ceremonies to go ahead and cancelled such tributes for her own birthday in April for the first time in her 68-year reign.
However, Buckingham Palace did release a new photograph of the royal couple to celebrate Philip's birthday, taken last week at the quadrangle inside Windsor Castle.
Philip, a former naval officer renowned for his sometimes brusque manner and occasional verbal gaffes, married the then Princess Elizabeth in 1947, five years before she became queen. He is now by far the longest-serving consort of any British monarch.
He stepped down from royal duties in August 2017 after completing more than 22,000 solo engagements but in recent years he has been increasingly rarely seen in public.
He did, however, break his retirement silence in April to issue a statement thanking those involved in the fight against COVID-19.
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