Emergency calls to Northern Ireland's Ambulance Service are regularly diverted to operators in Scotland, it can be revealed.
Dozens of 999 incidents every month are now being answered outside the region.
The number of calls rerouted to the Scottish Ambulance Service has risen by more than 80% year on year.
In 2019 over 550 emergency incidents were answered in Scotland, according to figures obtained by this newspaper.
DUP MLA Paul Givan said it was a "deeply worrying trend".
He said sending calls outside the region could lead to delays and disruption.
It comes as figures show many of the most urgent calls are not responded to inside the target eight-minute window, despite a new system brought in last autumn to tackle the problem.
When someone dials 999 their call is initially answered by the Public Emergency Call Service operated by BT.
The operator then attempts to connect the call to the Northern Ireland Ambulance Service's (NIAS) Emergency Ambulance Control.
However, in some circumstances calls are routed to the Scottish Ambulance Service as part of a 'buddy' arrangement.
These include where calls to the emergency control room exceeds its normal call handling capacity, or where an unplanned disruption to 999 call handling services occurs, through technical or other failures.
NIAS said the 'buddy' arrangement ensures an appropriate route for 999 calls to be redirected.
It said arrangements in place as standard across all UK ambulance services to ensure business continuity
However, the number of calls being answered in Scotland has risen sharply.
In the last five months of 2017, 29 calls were sent to Scottish operators.
But by 2018 this had risen to 306, rising further to 558 last year. Last September alone 98 calls were answered in Scotland.
The figures were released to this newspaper after a Freedom of Information request.
Mr Givan, who has raised the issue with Health Minister Robin Swann, said the reasons for so many calls being sent outside Northern Ireland must be identified.
He said: "These findings suggest a deeply worrying trend in the number of emergency calls from members of the public in need of medical assistance in Northern Ireland being rerouted to the Scottish Ambulance Service.
"In 2019, 558 such cases took place compared to 306 the previous year, and only 29 in the last six months of 2018.
"This demonstrates a stark and steady increase in contact lost between households and individuals in need of help locally and NIAS Emergency Ambulance Control. Rerouting these calls naturally increases the risk of delay and disruption in coordination of services.
"Whilst we recognise the need for effective 'buddy' arrangements with services elsewhere in the UK to ensure interrupted service in unforeseen situations, this should not be normal course day and daily.
"Reports of technical faults leading to calls being answered in Scotland extend as far back as 2015 and the root causes need to be identified and rectified."
In recent weeks concerns have been raised over ambulance cover in the Newry, south Down and south Armagh regions.
The Ambulance Service estimates it needs 335 additional staff to meet targets.
Mr Givan added: "It is also vital that an evidence-based approach is adopted to staffing in the NIAS and ambulance control in particular in order to ensure appropriate capacity to meet service demand across Northern Ireland.
"Covid-19 has already placed our dedicated ambulance crews and staff under additional operational pressure and they deserve to have every resource available to ensure they can carry out their roles in the most effective way.
"It is imperative that the Minister of Health moves to address these concerns as a matter of urgency."
NIAS said: "All UK services have buddying arrangements in place with neighbouring ambulances services.
"This is considered best practice.
"NIAS has a 'buddy system' with the Scottish Ambulance Service. This ensures continuity of service in the event of 'call surges' or interruption to the telephony network. This is a reciprocal arrangement and NIAS will take calls for Scotland.
"When this happens the Scottish Ambulance Service takes the call and it is passed to NIAS's command and control system via an electronic gateway for an ambulance to be despatched."