Ambulance crews in Northern Ireland have failed to respond to almost two-thirds of emergency call-outs within the target time.
Just over 37% of category A calls - where there is an immediate threat to life - met the eight-minute deadline.
In almost 700 cases last year, it took more than an hour for crews to arrive.
Figures seen by this newspaper show that the proportion of emergency calls being responded to in the target time has decreased.
SDLP MLA Dolores Kelly said she feared lives would be put at risk. "This is terribly worrying and is evidence of another crisis in our health service," she said.
The Northern Ireland Ambulance Service (NIAS) said it was working to ensure the most clinically urgent calls are prioritised.
It blamed staff shortages, increased demand and financial pressures for delays.
NIAS uses a call category system so patients receive a response appropriate to their needs.
Until recently, these were classed as categories A, B and C, but a modified system was introduced in November.
Category A calls are described as "the most serious calls, where there is, potentially, an immediate threat to life".
Official guidelines say these should be responded to within eight minutes. However, figures show NIAS is failing to meet that target in most cases. In the 12 months to last April, just 37.2% of category A calls were responded to in eight minutes.
That compares to 45.2% in 2017/18 and 51% in 2016/17.
Separate figures, for the 12 months to December 31 last year, show the situation has worsened still, dropping to 32%.
Last month the Belfast Telegraph reported how NIAS had apologised to a woman who waited nearly three hours for an ambulance after breaking her hip.
Ambulance officials admitted the ordeal endured by the 65-year-old, who was forced to lie on "ice-cold tiles" while waiting, was unacceptable.
Ms Kelly added: "These delays could put lives at risk.
"I have spoken to Ambulance Service staff and one issue is that too many crews are backed up in Accident and Emergency departments. They can be there for hours at a time and there is no one there to hand a patient over to."
The NIAS said ambulance response times have been increasing since 2012.
It blamed increased demand, staff shortages and reconfiguration of acute services involving longer ambulance journeys.
It also cited increased turnaround times at emergency departments because of wider unscheduled care pressures.
It said: "NIAS continues to take a range of actions to mitigate against these contributing factors to ensure the most clinically urgent calls are prioritised.
"In the context of increasing response times, NIAS developed a proposed new clinical response model, which after public consultation and departmental approval, was introduced in November 2019.
"This model aims to ensure that those patients who are sickest will receive our quickest response and that all other calls to the 999 system will receive a response appropriate to their needs.
"This resulted in a new set of call categories and associated response time standards being introduced from November 2019 which replace the previous category A, B and C calls."
NIAS has also trained more paramedics to address problems with staff shortages.
The NIAS said more than 300 additional operational staff will be required to meet demand and fully achieve response time targets.