Craigavon Area Hospital in Co Armagh stopped receiving ambulances carrying patients with non-life-threatening conditions on Sunday night due to severe capacity issues in its emergency department.
At one point there were 108 patients waiting in A&E, 32 needing hospital admission.
However, the hospital, which had 123 Covid-19 inpatients last night, only had three available beds.
The boss of the Southern Health and Social Care Trust said it was “exceptionally close” to declaring a major incident alert.
Shane Devlin said the health service in Northern Ireland was “on the edge”.
The worsening situation within the region’s under-pressure health system comes amid increasing Covid-19 transmission rates, particularly among young people.
Responding to the incident in Craigavon, the British Medical Association called on the Stormont Executive to revisit the issue of vaccine passports as a priority.
Making certification a legal entry requirement for hospitality venues has been credited with driving up vaccination rates among young people in the Irish Republic.
North of the border, the Executive has recommended that nightclubs and other venues carry out Covid entry checks and an official app has been developed to enable people to show proof of their vaccine status.
However, the administration has stopped short of making it a legal requirement of entry.
The issue has divided the five-party coalition, with the SDLP and Alliance Party calling for legally enforceable Covid passports for nightlife venues.
BMA Northern Ireland Council chair Tom Black said: “We have not even entered the winter months when traditionally we see our hospitals under tremendous pressure.
“Hospitals and indeed our whole health service was already struggling, but Covid and the added pressure from an increased number of patients is simply too much.
“We need to address these issues now. While vaccines are doing the job of keeping many people out of hospital, we also need to see masks being worn, contacts limited where possible and for everyone to come forward for boosters and flu jabs. The Executive should also revisit the use of Covid passports as a priority.”
SDLP leader Colum Eastwood accused the two main Executive parties – the DUP and Sinn Fein – of political “cowardice” and he warned that Northern Ireland could be facing fresh restrictions if certification was not introduced urgently.
The DUP and Sinn Fein have expressed doubts over a mandatory system, indicating their preference for a “partnership approach” with the hospitality industry.
“The message from our health service has been clear – they are on the brink of collapse and want to see urgent mitigations introduced, including the immediate introduction of vaccine certifications to take the pressure off staff and allow patients to get the care they need,” said Mr Eastwood.
“Health Minister Robin Swann (Ulster Unionist) has previously warned that our hospitals will have to turn people away if the situation worsens and we saw that play out last night at Craigavon Area Hospital.
“Sinn Fein and the DUP have thus far shown stunning political cowardice on this issue, a total abdication of leadership. They are unwilling to take the tough decisions needed to protect our health service and the public. This isn’t easy, since calling for the introduction of vaccine certifications myself and party colleagues have been harassed and protested against but our primary concern remains protecting people.
“We now face a stark choice, time is running out and this decision should have been made weeks ago. To protect our health service, so that cancer sufferers, people in need of urgent operations and others in desperate need of medical attention get access to the treatment they need, we will have to introduce vaccine certification or further restrictions to stop the spread of the virus.”
However, DUP Agriculture Minister Edwin Poots said he was not in favour of compulsory certification.
“I don’t support that at all,” he told BBC Radio Ulster.
“I am very clear that vaccination is the best means of creating normality and it has allowed us to create a significant normality once again in terms of our day-to-day lives. And I think that, you know, people should get vaccinated and I’ll encourage and cajole and pressurise and have done that with people who I know who have been hesitant.
“But forcing people to do something isn’t going to work. It’s not going to get those last few over the line. Over 90% of people are vaccinated and we need to continue to encourage the remainder to get vaccinated, but forcing them is not going to work.
“And I don’t think that creating a two-tier system for so many things is something that I would be prepared to accept.”
The ambulance divert at Craigavon was ended at 10pm on Sunday.
Southern Trust chief executive Mr Devlin, who has responsibility for the hospital, said the “vast majority” of the “exceptionally unwell” Covid-19 inpatients they were admitting were unvaccinated.
He told the BBC the trust was “exceptionally close” to declaring a major incident on Sunday.
“The system that we have, particularly with the level of Covid that we have in our hospitals at the moment, the system is absolutely on the edge in Craigavon and Daisy Hill (hospital) and the other trusts,” he said.