Senior royals are congratulating Prince Philip as the husband of Queen Elizabeth II celebrates his 98th birthday in private.
In a tweet Monday, the royal family wished Philip "a very happy" birthday.
The British Army planned to fire gun salutes in Philip's honor. The Army tweeted that it is "determined to make it a good one."
Some of Philip's children and grandchildren also posted tributes.
Philip has officially retired from public life although he is still occasionally seen with the queen at official events. He skipped President Donald Trump's visit.
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He still drives a horse-drawn carriage on private grounds from time to time but has stopped driving cars since a serious car accident in January. He gave up his driver's license at the time.