The singer broke the news to fans on social media Tuesday, saying the tough call became clear after his performance Saturday at the Glastonbury Festival in the UK.

Lewis -- who suffers from Tourette syndrome -- has often struggled with tics during his performances, with Glastonbury being the latest example ... but as usual, the massive crowd showed support by singing the song for him while he pulled away from the mic.

Lewis says, "The truth is I'm still learning to adjust to the impact of my Tourette's and on Saturday it became obvious that I need to spend much more time getting my mental and physical health in order, so I can keep doing everything I love."

No word on when he plans on taking the stage again ... but he ends by sending his love to all his fans -- "Playing for you every night is all I've ever dreamed of so this has been the most difficult decision of my life. I'll be back as soon as I possibly can. ❤️"