More than a quarter of ambulance staff in Northern Ireland were unavailable for work during August and September.
A total of 7,639 shifts were planned in a seven week period, but only 5,456 were filled.
It meant 2,183 shifts (around 28%) were left without cover.
The figures were revealed by Health Minister Robin Swann after an Assembly question from Sinn Fein MLA Orlaithi Flynn.
The West Belfast MLA asked Mr Swann to detail the working and required ambulance crews for August and September.
Mr Swann's answer provides the resource cover for accident and emergency ambulances, which have a crew of two, and rapid response vehicles (RRVs) - with one paramedic - for August and the first three weeks of September.
A breakdown of the figures shows more problems were encountered filling the RRV shifts.
A total of 5,785 A&E crew shifts were planned across all five health trusts in the seven week period but 4,651 were delivered - leaving a 19% gap.
Meanwhile, 1,854 rapid response vehicle shifts were planned but just 805 were delivered - a 57% deficit.
The Northern Trust had a 23% total gap between planned and actual crew shifts in August and September, with 37% in the Southern Trust, 33% in Belfast, 38% in the South Eastern Trust and 15% in the Western Trust area.
Mr Swann said the Ambulance Service (NIAS) are taking various steps to address the shortfall.
These include managing calls dynamically across the region, extending rapid response paramedic hours, using non-emergency crews, employing private ambulance services and providing telephone support to callers.
"While NIAS are continuing to recruit and train additional staff, I have made it clear that only significant additional investment will address these ongoing issues," Mr Swann said.
Ms Flynn said: "Over the course of recent weeks, Sinn Fein delegations have held meetings with paramedics, the head of the Ambulance Service, trade unions and the health trust.
"A pressing concerning that was emphasised by all was the limited personnel and resources.
"These issues, compounded by the ongoing health pandemic, has resulted in immense strain being placed on the service.
"I raised the concerns of the Ambulance Service directly with the Health Minister.
"Giving the worrying figures noted by the Health Minister of the limited capacity of Rapid Response Vehicles and ambulances during recent months, it is crucially important that the Minister urgently intervenes to provide maximum support to NIAS and ensures that the service is properly resourced."
NIAS warned in July that it was facing increased challenges in maintaining planned levels of cover across all of its divisions in Northern Ireland, meaning that response times to some calls would be delayed.
It outlined a range of reasons for the drop in staff cover and delayed response times.
This included increased use of NIAS services over recent years without an increase in staffing levels, 20% of frontline staff being absent due to ordinary sickness or reasons related to Covid-19, annual leave commitments, assaults on staff and similar issues facing Trust staff.
NIAS chief executive Michael Bloomfield has appealed for support from politicians to ensure that ambulances remain available to respond to life-threatening emergencies.