Driving Home For Christmas hit-maker Chris Rea is thought to be in a "stable" condition after he collapsed on stage.
The singer-songwriter from Middlesbrough had been performing at the New Theatre Oxford when the incident happened on Saturday, with one audience member describing him "falling into a clump".
Darren Fewins, who had been watching the show with his wife, told the Press Association the 66-year-old musician had been on stage for 40 to 45 minutes before he fell backwards.
"He was standing by the microphone and he had the guitar in his hand and he kept shaking his left hand as if there was something not quite right with it," the 46-year-old said.
"He walked forward to the microphone and he was trying to grab the microphone with his left hand, and it was swinging from side-to-side.
"I thought at first there was something wrong with it and he was trying to keep it steady. He then took three or four steps backwards.
"Then he just collapsed on the stage backwards. There was no-one there to catch him, he just fell into a clump.
"He was on his back for about two minutes before they brought the curtain down and then that was it."
Rea is currently on his The Road Songs For Lovers live tour and had been due to perform at Brighton on Sunday and Bournemouth on Tuesday.
The Middlesbrough-born musician was diagnosed with pancreatic cancer and had his pancreas removed in 2001.
A spokeswoman for South Central Ambulance Service said: "We were called at 9.30pm to attend to an incident in George Street in Oxford.
"We had an ambulance crew on scene and one patient has now been taken to hospital."
When asked about the condition of the person, she said they were "stable".