The Ambulance Service has admitted that one in six of its shifts cannot be covered because of staffing problems.
Almost 5,000 gaps appeared in rotas in just nine months due to a shortage of emergency crews.
Staff vacancies, rostered leave and sickness absences all led to reduced cover.
The percentage of unfilled shifts has almost doubled - rising from 8.2% in 2017/18 to 15% in the current year.
The figures were released by Health Minister Robin Swann after an Assembly question from DUP MLA Paul Givan.
Mr Givan said: "These figures are unacceptable and will cause public concern."
Between April 2017 and December 2019, a total of 13,084 ambulance shifts were not covered - 13 a day on average.
A breakdown by year shows:
Mr Givan said: "My question was prompted by the experience of a family within my constituency.
"On each occasion there has not been adequate cover for shifts within the Ambulance Service the public are at increased risk."
A separate Assembly question from Mr Givan found huge overtime costs to plug gaps in the Ambulance Service's rotas.
Some £18,083,000 was spent to cover additional hours in the last three years.
Mr Givan added: "I welcome the Executive putting our health service at the top of its priorities and reform must be made across a wide range of issues.
"Progress has been made in dealing with the pay dispute but health service reform is a major but necessary task and will only be achieved through support from all sides of the Assembly."
Last summer ambulance crews had to be drafted from the Republic of Ireland to help cover major staff shortages.
During one weekend in August, the Northern Ireland Ambulance Service was down by about 20 crews - roughly a quarter of its normal workforce.
According to the Ambulance Service, reduced levels of cover are mainly due to factors such as staff vacancies, rostered leave and sickness absences.
It said that whenever it has not been possible to provide full cover, contingency plans have been instigated to mitigate against an adverse impact on patients.
"Calls received in ambulance control are prioritised to ensure that the quickest response is provided to the most seriously ill or injured," a spokesman said.
He said demand has increased significantly in recent years without a corresponding increase in ambulance resources.
The spokesman said the service is working to address staff shortages and fill existing vacancies with ongoing regional recruitments and training.
He said over 300 additional operational staff will be required to meet demand and achieve response time targets.
Ambulance Service chief executive Michael Bloomfield acknowledged the problems caused by unfilled shifts on the public and staff.
"It is a source of regret to me whenever ambulance cover is depleted due to lack of available resources and the potential impact this may have on the community we serve," he said.
"I am mindful that as a result of reduced staffing levels some patients wait longer for an ambulance than would otherwise be the case.
"We continue to prioritise the most urgent calls, but regrettably some lower urgency calls will wait longer and I apologise to patients for that.
"I would like to express my gratitude to those staff, on the frontline and in ambulance control, who continue to work tirelessly to ensure that an ambulance response is provided to those who have an immediate and life-threatening need."
In January it was reported that the Ambulance Service had spent £3.2m on private ambulances in 2018/19.
The ambulances - run by private firms and voluntary organisations - were sent to 8,514 emergency calls.
The Red Cross, St John Ambulance and the Order of Malta are among those used to provide cover.
Health union Unison said increasing private ambulance use was concerning and was indicative of problems across the health service.