Suspected stroke patients are waiting up to ten hours for an ambulance to arrive, an ITV News investigation can reveal.
It comes as response figures for 'amber' calls show thousands more people in Wales are waiting more than an hour for an ambulance compared to a year ago.
Between October 2016 and August 2017, that figure had increased by 48.6% to 22,549.
The total number of amber calls to the ambulance service between the two ten month periods increased by 3.6%.
What is an amber call?
In October 2015, the ambulance service changed its response model. The change saw the introduction of a traffic light system. Red for life threatening calls, amber for serious bit not life threatening and green for less serious incidents. Under the new model, only red calls have a target time.
But the majority of calls to the ambulance service are graded as amber, and have no target time. This category includes medical conditions such as stroke, assault, lacerations, poisoning and childbirth.
Under the latest figures obtained by ITV News, suspected stroke patients are waiting up to 10 hours for an ambulance to arrive.
Public information campaigns highlight the need to act 'FAST' when people are suspected of suffering a stroke. Four hours is regarded as the essential window in which intervention is required in order to ensure the best outcome for a patient.
On seeing the findings of the ITV News investigation, the Stroke Association in Wales said the figures were of great concern.
The figures have also prompted a political outcry with the Welsh Conservatives questioning how well monitored the model is and whether there needs to be 'course correction'.
You can watch the report here: