The Queen gave Prince Harry special permission to sport facial hair while wearing uniform on his wedding day.
It is customary to be clean-shaven when dressed in Army uniform.
Both the newly-appointed Duke of Sussex and the Duke of Cambridge wore the frockcoat uniform of the Blues and Royals during Harry's wedding to former Hollywood actress Meghan.
In a statement, Kensington Palace said: "Her Majesty The Queen gave her permission for Prince Harry to get married in his uniform."
It is understood that the permission covered the fact that Harry has a beard.
Historian Hugo Vickers said the permission may also have been granted because Harry has retired from active service.
"I think if you've retired from the Army you probably do need somebody's permission to wear your old uniform," Mr Vickers said.
"But I think the statement covers all those possibilities."
British Army rules do not usually allow beards, except in rare circumstances, such as because of skin complaints, or for religious reasons.
Pioneer Sergeants are the only rank allowed to have a beard, although members of the Special Forces can when behind enemy lines.
Harry is, however, no longer a serving officer and therefore military rules may not apply to him.
A bearded royal bridegroom is a rarity. Indeed none of the current Royal Family sported facial hair on their own wedding days.
But the Queen's grandfather, George V, did have a beard when he married Mary of Teck.
Harry's uniform revealed that some time after leaving the Army in 2015 he was promoted to the rank of major, an honour that was bestowed by the Queen.
The prince wore a frockcoat uniform of the Blues and Royals ranked to major, with large gold embroidered crowns.
It is the Queen's prerogative that senior members of the royal family are promoted in line with their peers because they hold royal military appointments.
Harry left the forces in June 2015 and was first spotted with a beard in September the same year.
Australian Associated Press
