Forty brand new ambulances bought at a cost of £4m are off the road because they cannot cope with speed bumps.
The £100,000 vehicles are being stored behind ambulance stations
The Mercedes vehicles were bought last year by the Tees, East and North Yorkshire Ambulance Service Trust (TENYAS) but they have never been used.
The problem is the hydraulic lift fitted to back which has been placed too low and hits the road when driven over the traffic-calming road humps.
Earlier this month, TENYAS was branded one of the worst services in the country, according to government figures, after it was awarded no stars.
While the £100,000 ambulances await modification, the vehicles are being stored at the back of ambulance stations.
In the meantime, the service is having to rely on older vehicles which require more maintenance and spend more time off the road.
The problem is the tail lift fitted at the back of the ambulance
Union official Glen Gears from Unison said there were no staff representatives on the group that drew up the specification for the new ambulances.
He added: "Particularly around York and Hull there are lots of speed bumps and I suspect that this wasn't highlighted.
"Had it been so, perhaps it could have been avoided.
"Crews want to have pride in their vehicles so it is demoralising and really frustrating."
In a statement, TENYAS said the vehicles were due to be modified and should be on the road by the end of September.