The Duke of Edinburgh's official cause of death has reportedly surfaced, and it's simply stated as "old age" ... this according to the Daily Telegraph, which says it obtained his death certificate.

Now, we know what you're thinking ... what the hell does that even mean??? Welp, according to experts from across the pond -- citing old age is acceptable for someone over 80 who had one doctor look over his care for years ... and if there's no other identifiable disease or injury responsible for the death.

So, the obvious issue is Queen Elizabeth's husband had a number of ailments in his later years ... including a heart scare just months before he passed. Still, "old age" is what Buckingham Palace is going with and they kinda make the rules.

As they say ... it's good to be the Queen.

Seriously, though, in the U.S. ... it seems like the kind of thing we'd list as "natural causes."

As we reported ... 99-year-old Philip died last month, and the Royal Family held a grand funeral procession for him. Prince Harry flew in from the States for the ceremony and got some face time in with his brother and father ... the first time he'd seen them since his and Meghan's bombshell interview with Oprah.

One last thing about Phil's death cert -- it lists his full name, which is a mouthful ... even for a Royal. It reads ... His Royal Highness The Prince Philip, Duke of Edinburgh formerly known as Prince Philippos of Greece and Denmark formerly known as Philip Mountbatten.

His occupations were Naval Officer, and Prince of the United Kingdom.