The 22-year-old performer said he once met the future King of England after performing at a festival.
The Sun newspaper has an exclusive extract from his new book, My Story So Far, where he revealed the funny story.
He said: 'One of my favourite early shows was Radio 1's Big Weekend in Bangor, North Wales. I had a ball onstage and I came off all sweaty, with my top off.
'I saw this guy in a baseball cap and his beautiful girlfriend, waiting for me. I looked under his cap and I froze.
'I know that face. They just looked like a very normal, happy couple - but it was the future king and queen, Prince William and Kate Middleton.'
He added: 'William was really dressed down. Kate looked gorgeous in a nice dress. He put his hand on my shoulder and I cringed. I was so sweaty, like eurgggh.
'He made a little joke about my trousers being so low, saying he thought they were going to fall off halfway through my performance and said he loves going to festivals but he goes incognito.
'His aide came out with a box of Soleros and there was only four in there but there were five of us in the room. William offered me one and I was like: 'No thanks.''