Paul Mason – who once weighed 980lbs but ha slimmed down to 560lbs – was feared to have suffered a heart-attack.
Medics called to the scene found the 50-year-old face down and moaning in agony in undergrowth outside a Sainsbury's supermarket in Warren Heath, Ipswich, but were unable to move him.
Firefighters arrived to help and a specially-strengthened ambulance designed for the hyper-obese was called in.
He was taken to Ipswich Hospital with serious head injuries and a suspected broken shoulder.
One eye witness said: “It was a real shock. He just seemed to lose control of his chair and it went off as if it had a mind of its own.
“At first I thought it was funny but after this huge man tipped out and just lay there motionless we knew it was serious.”
He was still being treated today.
Mr Mason is currently suing Britain's health service for failing to help him as his weight ballooned.
However, his care is believed to have cost British tax payers more than £1 million over his lifetime.