Kemp, who is the son of Spandau Ballet star Martin Kemp, said it wasn’t a word he even usually uses to describe someone.
But it slipped out when he saw the royal at the bash.
The 29-year-old opened up on ITV’s Loose Women, saying it happened after he hosted the Jubilee Platinum Party.
He said: “Afterwards they did have a party inside Buckingham Palace, it was an honour to be there. It kind of felt like they probably meant to invite my dad!
“Anyway I was there, I was in there and everyone was enjoying it, long day, you know, finished it. And what was so brilliant was that the Palace also invited all the camera crew and everyone was in there. It was really fantastic.
“As I was walking across the room, I’ve kind of like bumped in… Princess Catherine she was there, and all of a sudden I’ve seen her and she said, 'Oh Roman, oh you look great' because I had this nice suit on, and I was like, ‘You look really fit’.”
“I’ve never said that to anyone!” he told the panellists as they burst out laughing.
Kemp went on: “Honestly I was mortified, I was like, ‘I’m so sorry’.”
However, it seems his apology might have been a bit late, as he told the TV stars: “She was already gone!”
Watch: Roman Kemp asks the public if they ever give honest answers when asked how they are