Jack Whitehall "absolutely died" on stage in front of Prince Charles.
The 30-year-old comic admitted it was an "honour" to be selected to provide the entertainment for the 70-year-old royal's annual staff Christmas party but it was a very "weird gig" and none of his material went down well.
He recalled: "It was an honour to be asked to do the Christmas party of Prince Charles... he throws [his staff] a Christmas party each year... and I was the entertainment that year. It's the weirdest gig ever.
"I walk out, first thing that's weird is that they're sat in the front row, Charles and Camilla, in thrones... Also, don't sit in the front row because that's your get-out-of-jail-free card as a comic, [you pick on the front row], you ask them what they do for a living... I can't ask Prince Charles what he does for a living!
[It was a] small room, maybe 40 people, all of my material was terrible... I absolutely died."
And things only got worse for Jack as he overheard the prince suggesting to his wife, Duchess Camilla, that they opt for a very different form of entertainment in future.
He added on 'The Jonathan Ross Show': "And the worst bit was, when I was walking off because nobody was applauding, it was the worst review I've ever got, I heard Charles turn to Camilla as I was leaving the stage and he went 'I think next year we'll try a magician'... There goes the knighthood!"
Jack recently hosted the BRIT Awards and while he was "so excited" to get the chance to be a part of the show opener with Hugh Jackman, he was horrified in rehearsals when his part in the spectacle was considerably scaled back.
He said: "I got to join in with that. I love 'The Greatest Showman' I was so excited to be a part of that. Initially I was going to sing. They were like, 'Would you like to open the show with Hugh Jackman?' I was like 'Oh my God yes, that'd be amazing!'
"I thought I did [have a good voice] and they gave me that solo, the Keala solo in that song which is Whitney levels of high pitched, it's so difficult.
"I went off, I went to a sound studio, I recorded my voice, I laid it all done and I got to the rehearsal and they went 'We're going to have you dancing at the back now.' I got cut completely."
'The Jonathan Ross Show' airs on ITV on Saturday (16.03.19).