Singer Josh Groban suffered his worst nightmare onstage in front of 15,000 fans when a severe allergic reaction left him whispering.

The You Raise Me Up star was three songs into his set when he realized something was wrong with his golden voice.

He tells Access Hollywood Live, "I had 15 songs to go... It was like a nightmare... I had to level with the audience and say, 'All right guys, I could do one of two things: I could quit the show or you could give me 15 minutes and I'll be right back'.

"I stopped the show, I went backstage, I drank some tea, I warmed up slowly, I tried not to freak out and I came out, and note by note I got my voice back and I finished the show. It was a really great learning experience."

And, as for what caused the allergic reaction, Groban adds, "It was some smoothie and it had some nut or seed in it."