THE Ambulance Service has predicted a rise in the number of "less urgent" 999 calls as a heatwave lands today and tomorrow.
It said patients may be asked if they can make their own way to hospital, if resources are stretched.
Today is expected to see some of the hottest temperatures of the year, with 30C or more predicted in some counties, particularly Fermanagh and Tyrone.
Night-time temperatures will stay in the mid to high teens, making sleep uncomfortable for some.
The Northern Ireland Ambulance Service has said it is expecting an increase in calls.
It urged people to only call in an emergency.
The service also asked that patients make their own way to hospital emergency departments, if that would be quicker than waiting for an ambulance.
"While speaking to our control staff, the caller may also be asked at that stage if it is possible to transport the patient by other means with the aim of getting them to definitive care quicker when resources are stretched," a spokesman said.
"When calling 999 you will be advised that your call has been accepted and to only call back if there is a change in patient’s condition. Please do not call just to check on ambulance arrival as this creates difficulties in answering other calls.
"If you find you are waiting for control staff to speak to you, please be patient and do not hang up as you would lose your place in the queue. We will answer you as soon as someone is available.
"We want to re-emphasise that calls will be prioritised with less serious calls having to wait extended periods. We apologise for any delays you may experience but can assure you that we will get an ambulance to you as soon as we possibly can."
An extreme heat warning - the first the Met Office has ever issued - has been made for most of England.
Temperatures are expected to climb to 40C in some places, including London, today and tomorrow.
The highest-ever temperature recorded in Northern Ireland was 31.2C at Ballywatticock, near Newtownards in Co Down, in July last year.
In the Republic, a Status Yellow alert began at 6am yesterday will last until 9pm on Tuesday, with temperatures of up to 32C possible in places today.
The Met Eireann weather warning stated: “On Sunday and Monday exceptionally warm weather will occur over Ireland with daytime temperatures of 25 to 30 degrees and possibly up to 32 degrees in places on Monday.
“Night time temperatures will range from 15 to 20 degrees.”
Met Eireann said there was an increased risk of heat stress, high solar UV index and water-related incidents.
The NI Ambulance Service has appealed to people to stay out of the sun between the hottest hours of 11am to 3pm; avoid heavy exercise; wear light loose-fitting cotton clothes, and have plenty of cold drinks.
People who have to go out in the heat are advised to walk in the shade, apply sunscreen and wear a hat
The service said young children and animals should never be left in a closed vehicle, even for a few minutes.
It also warned against diving into quarry ponds or gravel pits because the unexpected cold can cause shock. The ponds can also contain hidden obstacles which are dangerous for swimmers.
