An ambulance mechanic suffered extensive crushing injuries when the vehicle fell on him while he was working on it.
The 22-year-old man suffered injuries to his torso and pelvis, according to ThisisBristol.
He was in the intensive care unit at Frenchay Hospital.
His injuries were described by Great Western Ambulance Service as life-threatening.
The man was in intensive care at Frenchay Hospital last night, with his family beside him.
The ambulance, owned by Great Western Ambulance Service, was being worked on at the Mercedes commercial vehicle specialist, the paper reported.
Paramedics saved vital minutes by using a stretcher from the ambulance involved in the incident to prepare the man for transfer to hospital.
A spokesman for City West Commercials said the incident had been reported in accordance with the company's protocol to the Health and Safety Executive.
A helicopter crew that included a medic and a doctor landed nearby, and rushed to treat the severely injured man.
"The rapid response team and critical care team from the helicopter used the stretcher in the back of the ambulance on scene to prepare the patient for road transfer, saving vital minutes. He was then transferred by road to Frenchay Hospital, with what paramedics on scene described as life- threatening injuries," an official told reporters.
The injured mechanic "was given powerful painkillers at the scene, and his pelvis placed into a splint," according to the ambulance official.