The Duke of York attempted to give his wishes to eldest daughter Princess Beatrice as she turned 31 on Thursday.
He posted a message on Twitter which read: “Happy Birthday Princess Beatrice, Thank you to everyone for the lovely birthday wishes!”
The Duke also added four pictures to the tweet – only they were of the wrong daughter.
The wrong snaps were of his other daughter Princess Eugenie – including one of her on her wedding day.
Eugenie’s birthday is not until next year on March 23, and she is currently 29-years-old.
Royal staff quickly spotted the blunder on HRH’s official Twitter account and removed it.
The message was soon replaced by four pictures of the right daughter.
But the gaffe didn’t go unnoticed by eagle-eyed royal fans, with some screenshotting the original tweet before it was deleted.
One uploaded the unfortunate tweet and wrote the caption: “Emmm this is a wee bit awkward?!?!”
Another joked: “I never know who is Eugenie and who is Beatrice – thank god I’m not alone and their dad have the same problem.”
Both Prince Andrew and mum Sarah Ferguson, 59, also posted sweet tributes on Instagram for their first born.
Fergie shared snaps from the York family album, and referred to her as “dearest Beatrice”.
Both Beatrice and Eugenie were born in London’s Portmore Hospital.
This is the same hospital that saw the birth of Prince Harry and Meghan Markle’s son Archie Harrison Mountbatten-Windsor.