The performance of the Welsh Ambulance Trust has been branded "appalling" and "unacceptable" in a report by an all-party committee of AMs.
Management failings were 'completely unacceptable'
The Assembly Audit Committee said the trust has failed to deliver acceptable standards of emergency response times.
It said south east Wales was hit particularly badly, with management failings and not lack of money blamed.
The ambulance trust said the report gave "cause for optimism" and was a "vote of confidence" in new management.
Performance was significantly worse than in England, and this was due to inefficiency and not a shortage of staff or finance, said AMs.
According to the report, the reasons for the ambulance trust's failure were many and varied but were caused by weaknesses in all areas of management practice and not by a lack of resources.
Problems with the trust included inadequate leadership, a confused internal structure and poor use of financial resources.
However, the committee also found that there were grounds for optimism that that these problems could be resolved over time provided key challenges were met.
Committee chair, Janet Davies AM, said the problems pre-dated present chief Alan Murray adding she believed improvements were taking place.
She said: "The management failings which led to the poor performance of the ambulance service were completely unacceptable and should be a salutary lesson to other public sector organisations in Wales and beyond."
She said that the committee was "shocked by the scale of the failings".
Ms Davies added: "The trust has had considerable resources at its disposal but has been inefficient in the way it has used them.
"We did find grounds for optimism that the service can be improved under the new team led by Alan Murray, but only if these key shortcomings are addressed."
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The trust has been beset by problems, with two changes in the post of chief executive over the last 10 months and AMs voting for a public inquiry.
The Welsh Ambulance Service NHS Trust said the report echoed findings of earlier reports by the auditor general and Healthcare Inspectorate Wales.
It said it also pointed to optimism and said it was "another vote of confidence in its new leadership".
Chief executive Alan Murray said: "We did not wait for those reports to be published before we started making improvements and we are already addressing all the major issues raised."
He said it was a "three-to-five-year project" but that immediate improvements were being made.
Mr Murray added his role was the "toughest job" he had been involved in.
Last week, the trust announced an improvement in is response times.
Its quarterly figures to December showed a 1.3% improvement across Wales in ambulances reaching emergency calls within the target time of eight minutes.
Only north Wales (62.9%) passed the 60% target, with an all-Wales average of 54.2% of calls being met within eight minutes.
The busiest region, south east Wales achieved 49.2% but the trust said locally, Cardiff, Merthyr and Newport were now hitting the 60% target.
In central and west Wales it was 55.4%.
But Mr Murray admitted that record demand on the service in recent weeks "would have an impact" on the next set of figures.