Three new ambulances worth a total of £240,000 have been left unused for almost a year because crews claim they are unsafe.
Staff are refusing to use the Avon Ambulance Service vehicles because they say a wall between the cab and the rear of the vehicles makes them dangerous.
They fear that crew in the back of the ambulances may not be able to alert the driver if a patient becomes violent.
But chiefs say the vehicles are safe and are used across the UK.
Terry Hayward, director of operations at Avon Ambulance Service, told BBC News Online: “We have a live system so that everything that happens in the back can be heard in the cab of the vehicle.”
Unison, the crew’s union, said it now accepted the communication system worked, but claimed other health and safety issues had yet to be resolved.