Lives are at being put at risk because fire crews are sometimes used when no ambulances are available, according to the Fire Brigades Union (FBU).
It is hoped a £16m investment in vehicles will help improve things
Mike Smith, the FBU's executive council member for Wales, estimated it had happened on around 90 occasions in south Wales over the past 12 months.
He told BBC Wales' Eye on Wales that one day somebody was "going to die".
But the head of the Welsh Ambulance Service said he was confident its performance was already improving.
Fire engines are not the only emergency vehicle sometimes used as substitute ambulances, the programme found.
During a two month period last summer, the South Wales Police Federation identified 57 incidents when officers had difficulty getting an ambulance to the scene.
And on 25 occasions the police ending up taking people to Accident and Emergency.
In February, a man stabbed outside a Maesteg pub was taken to hospital in a fire engine because the area's three ambulances were busy.
Mr Smith said instances of fire engines being used were too frequent.
"It is a huge, huge problem. I think that somebody is going to die one day," he warned.
"They're either going to die because the medical intervention is not there or somebody's going to die because the fire appliance is actually doing something it shouldn't be."
The Welsh Ambulance Trust has been beset by problems in recent times, with two changes in the post of chief executive over the last 10 months and AMs voting for a public inquiry.
Last week its performance was branded "appalling" and "unacceptable" in a report by an all-party committee of AMs.
However, recent figures showed an improvement in the number of ambulances reaching emergency calls within the eight minute target time, although the figure was still below the 60% bar set by the Welsh Assembly Government.
Welsh Ambulance Service NHS Trust Chief Executive Alan Murray said he had no doubt that his crews would achieve the 60% target by March 2008.
Mr Murray acknowledged in the past performance had not been up to scratch.
"When someone needs an ambulance they don't want a fire engine or police car - and that's absolutely right.
"I've spoken to my fire and police colleagues to thank them.
"We've been explaining the situation to them so that they understand the need for a bit more forbearance until we get this right."
Eye on Wales is on BBC Radio Wales at 1800 GMT on Monday 12 March.