The sudden closure of the casualty service at Lagan Valley Hospital will result in lives being lost, it has been warned.
The South Eastern Health & Social Care Trust has announced the A&E at the Lisburn hospital will close between 8pm and 9am every day from Monday.
Patients who require emergency treatment at night-time will be expected to travel to other hospitals.
Kieran McCarthy, a member of the Stormont health committee, said he has no doubt lives are being put at risk by the decision.
"This is horrendous, this is wrong," he said. "I am very disappointed they are pressing ahead with these changes particularly in light of the changes that are due to happen at the City Hospital in Belfast.
"I don't think our A&Es are going to be able to cope."
Chair of the health committee Jim Wells also expressed concern at the speed at which the changes are being implemented.
"I only hope contingency plans have been put in place at the other hospitals which will take the patients from Lagan Valley," he said.
Health Minister Edwin Poots revealed opening hours at the A&E were likely to be scaled back in a shock announcement to the health committee last week.
He blamed the change on difficulties recruiting junior doctors to work in the department and said he expected most patients would travel to Craigavon Area Hospital or the Royal Victoria Hospital.
However, Mr Wells said: "They are hanging from the rafters in the A&E at Craigavon at the weekend so time will tell whether these other hospitals can cope."
A spokeswoman from the Trust said: "Despite extensive efforts to sustain services as normal, the Health and Social Care Board has agreed with the trust that temporary changes proposed by the trust are necessary.
"The Board has been working with all Trusts including Northern Ireland Ambulance Service to ensure that the changes can happen smoothly and safely."
In a shock announcement, the Stormont health committee last week, Health Minister Edwin Poots revealed opening hours at Lagan Valley Hospital's A&E will be reduced from next month.
He blamed the changes on a shortage of junior doctors.
He also revealed the number of A&Es in Belfast are likely to be cut to two and said City Hospital is in the firing line. Mr Poots said the changes are vital for patient safety.