The Driving Home for Christmas singer, who is 66, collapsed mid-song as he performed at the New Theatre Oxford.
He was reportedly taken away in an ambulance.
Fans began tweeting about the incident as the gig took place.
One wrote: "The evening had been brilliant up till now with Chris on top form but Chris Rea has just collapsed on stage at the New Theatre Oxford where I'm watching him.
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"Fell backwards in a clump on the stage. Really doesn't look good. I'm quite upset."
Another added: "I'm at the New Theatre Oxford. Moments after I took this photo Chris Rea collapsed on-stage.
"Fell backwards after shaking and struggling to hold the microphone."
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A third continued: "I was at Chris Rea's show tonight where he collapsed on stage and was taken away in an ambulance. We are all praying he is ok."
The incident comes just a year after the English star suffered from a stroke.
The Middlesbrough-born musician also battled pancreatic cancer in the early 2000s.
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Chris recently talked health battles to Gazette Live.
He said: "I just keep having the worst luck in the world. I'm still getting the effects from 15 years ago – because I didn't die!"
The star recently gave up smoking, and admitted singing his first song without a cigarette was "very strange".