DUP MP Sir Jeffrey Donaldson has said his party still needs legal assurances on the backstop to support Theresa May’s deal.
Sir Jeffrey said he and colleagues would “work day and night” to get such assurances.
“If those issues are not adequately addressed than I don’t think this agreement is going to go through,” he told BBC Radio Ulster.
When asked what the party specifically required, Mr Donaldson declined to be drawn, but he did highlight paragraph 50 of the EU/UK joint report on Brexit from December 2017 that referenced a strong role for Stormont in the operation of the backstop.
The paragraph was not subsequently included in the Withdrawal Agreement.
The interview came after days of speculation that the Government may move to strengthen the role of the Assembly in domestic law in order to win DUP support for the Brexit withdrawal treaty.
“I am not going to negotiate with the Government over the airwaves, all I will say is that last December 2017 paragraph 50 of the joint report, which we insisted on having included in that report, gives safeguards to Northern Ireland,” he added.
“Those safeguards were removed in the Withdrawal Agreement. We need that issue addressed by the Government and that’s what we are working towards.”
Sir Jeffrey said the Government needed to translate “verbal commitments” on the issue into “law”.
“Our priority is preserving the Union and getting an outcome that is economically good for Northern Ireland,” he added.
“We do not want a no-deal outcome, that is not the preferred outcome of the DUP. It is not what the DUP is working towards.
“We are working towards an agreement that we can support that delivers Brexit and is good for Northern Ireland and good for the United Kingdom as a whole.
“What’s on the table at the moment is potentially harmful to Northern Ireland, which is why we want the legal assurances that we are seeking in relation to the backstop before we can commit to support this deal.
“That is our objective – clear, simply and absolute.”
However, Sinn Fein’s Stormont leader has said that Sinn Fein would reject any move to bolster the powers of the devolved administration in relation to the backstop’s operation.
Michelle O’Neill made her party’s stance clear on Thursday.
“There will not be any situation where there will be a veto handed to this Assembly,” she said.
The British Government has already committed to giving Stormont a consultative role before a decision is made to either enter the backstop or extend the implementation period.
And, if the backstop does come into effect, the Stormont Assembly and Executive would be asked to consent to any new or amended EU laws applying to the region.
Mrs O’Neill said: “Of course there can be a consultative role, that was set out in the Withdrawal Agreement as it stands, but there cannot be any veto afforded to the DUP or anybody else in this situation.”
The European Council’s conclusions document, agreed by the EU27 leaders, warns that any unilateral commitment or statement should be compatible with the letter of the Withdrawal Agreement, meaning that any promises made by the Prime Minister to the DUP cannot undermine the backstop.