A group set up to oppose plans for a controversial waste incinerator facility in County Antrim has said it is considering a legal challenge.
On Wednesday, the facility in Mallusk was given planning approval, in the absence of a minister, by the Department for Infrastructure (DfI).
A campaign group that opposes the plans said it was disappointed by the "fundamentally flawed proposal".
It had been turned down in 2015 by then environment minister, Mark H Durkan.
Last year, the waste management group behind the scheme, Arc21, appealed against his decision.
The Planning Appeals Commission recommended approval. The department has now agreed, saying it was "in the public interest to make the decision without further delay".
While departments normally exercise their powers subject to the direction and control of ministers, it added, the power to grant planning permission for regionally significant development proposals is vested in the department.
"This is a strategically important project for the region and therefore the department considered that it is in the public interest to take this decision without further delay," said a DfI spokesperson.
SDLP deputy leader Nichola Mallon said she had written to the head of the Northern Ireland Civil Service (NICS) to ask questions on "process, justification and time" regarding the decision.
I've written to Head of NICS asking 4 urgent answers on process, justification & timing of this irregular decision on Hightown Incinerator pic.twitter.com/mu9WA7q0t8— Nichola Mallon (@NicholaMallon) September 14, 2017
In a letter to David Sterling, seen by the BBC, she asked for "urgent clarity and full details of the processes followed, the rationale and justification for this decision, what other civil servants were involved in this decision besides the permanent secretary, on what authority this decision was taken."
The letter also asked: "Was the department in receipt of any related legal communications or threat of legal action sent by or on behalf of the applicant and why was this decision taken given that it has issued in the most positive context to date, of progress in respect of the talks and reformation of a devolved executive with mandated ministers in place?"
The NI Executive Office confirmed receipt of the letter.
The facility plans to deal with black bin waste from six super councils.
It has been controversial from the start with residents of Glengormley opposing it because of concerns about traffic and the effect on property prices.
More than 3,500 objections were lodged when the planning application was first made.
A group set up to oppose plans for the plant, NoArc21, said it was disappointed at the decision.
Colin Buick, chair of the NoArc21 campaign, said news of the planning approval came out of the blue, but that his group was already planning a legal challenge.
"It would be our intention that we'll be taking the legal route - a judicial review," he said.
"We are also extremely concerned that the decision has been taken without a minister in place given the fact that all political parties have supported our campaign.
"The fact that 4,002 letters of objection and one petition of objection with 836 signatures were submitted in relation to the planning application shows the strength of feeling among the local community.
"Despite today's announcement, local people will continue in our opposition to this flawed proposal and NoARC21 will be carefully considering our legal options," he said.
The Department for Infrastructure took over planning responsibilities in the re-organisation of Stormont departments and its minister has the final say on major applications.
There has not been a functioning Northern Ireland Executive since January.
'Anger, disappointment and confusion'
Speaking on BBC's Good Morning Ulster on Thursday, former environment minister Mark H Durkan said he was "surprised at the timing of the decision".
"There a lot of anger and disappointment, also a lot of confusion, as to how a decision of this magnitude can be made in the absence of a minister and executive.
"This should be more than a ministerial decision. In my opinion it should have been subject to an executive decision," he said.
Despite planning officials recommending the application for approval, Mr Durkan said he rejected it because he felt it was "counter intuitive to the direction of travel I wanted to see as environmental minister, and that's a road towards zero waste".
However, the Planning Appeals Commission report said it took the view that that decision to refuse the application was "unjustified."
It also said there was no evidence that the facility would not meet the regulatory requirements and there was "inadequate evidence that it would be a risk to human health".
Mr Durkan said he stood by his original decision, adding: "I can sleep in my bed at night knowing I did the right thing".
All five of Northern Ireland's main parties have criticised the decision.
The Department for Infrastructure said it took the decision to approve the plan having "carefully considered and agreed" with the PAC report.
"It is clear from all the evidence presented in the planning application and debated at the PAC that there is a demonstrable need for this development," it said.
"Northern Ireland needs new infrastructure in order to support a resource-efficient, self-sufficient, legitimate system of waste management."