- Aug 10, 2003
- United Kingdom
She was at first called Lady Louise Windsor officially.
Announcement of the christening of Lady Louise Windsor | The Royal Family
I don't think it was rescinded - simply that they believed a double surname should be cumbersome.
Louise and James are both registered as Mountbatten- Windsor on their birth certificates. And that is how they have both entered schooling. However Louise name tag at school definitely just said Windsor, but her full surname was used on certificates ect. She competes in the horse riding competitions as Mountbatten-Windsor.
So they appear to use it as both.