Nigel Dodds has again insisted that there must be no "border down the Irish Sea" but the DUP deputy leader appeared less categorical on the potential for the UK remaining in the customs union.
His comments came as British Prime Minister Theresa May moved to dismiss weekend reports that she would bow to parliamentary pressure and remain in a customs union with the EU after Brexit.
Pro-European Tory MPs are set to voice their concerns over withdrawal from the customs union on Thursday. Advocates of remaining in a customs union believe it would enable the Irish border to remain frictionless.
In a statement yesterday, Mr Dodds reiterated his belief that the United Kingdom will exit the EU as "one nation".
He said that since the Brexit negotiations began there been a series of "leaks’ and rumours" which were primarily designed to "exert pressure on the UK government".
"The government has restated its commitment to leaving the customs union and it is in the European Union’s interests as well as the United Kingdom’s to secure a positive outcome," the North Belfast MP said.
"Behind the sabre rattling there remains the opportunity to secure such an outcome and the UK government has been positive and proactive in putting forward proposals to achieve this."
Mr Dodds said it was fundamental that the post-Brexit arrangements "apply to the entirety of the United Kingdom" – but notably he did not rule out continued membership of the customs union.
"There can be absolutely no question of any border down the Irish Sea," he said.
"A new customs partnership and an ambitious free trade deal between the UK and EU can deliver the best outcome for all sides – that is where the focus should be rather than reheating old arguments around the customs union."
Sinn Féin Brexit spokesman David Cullinane said the UK's proposals for maintaining a frictionless border through a technological solution were "were utterly useless and unworkable".
"The fact remains that if the north leaves the customs union and single market, and it no longer remains under the EU legal framework, there will be border checks," he said.
"This is why Sinn Féin has consistently called for the north to have a form of special status within the EU if Britain leaves the customs union and single market – this is the only practical solution."
SDLP leader Colum Eastwood challenged Mrs May to "finally face down the minority of Brexit extremists in her own cabinet".
"It is a fundamentally dangerous idea that we now face the threat of being dragged out of the European customs union simply because this British government doesn’t have the courage to face down the small minority of extremists in its cabinet and on its backbenches," he said.
“It is decision time for the British Prime Minister Theresa May – allow the future economic interests of these islands to be dictated by the extreme Brexiteers Boris Johnson and Liam Fox or finally face them down and get rid of them."
Mr Eastwood said "every credible" business organisation in Ireland and Britain supported continued alignment with the customs union and the single market.
“It remains the only way to prevent a hardening of the border in Ireland and it remains the best way of protecting the economic interests of both Ireland and Britain.”
Alliance deputy leader Stephen Farry said the "most sensible way forward" was the UK's continued membership of a customs union with the EU.
"There is already a clear majority within the House of Lords for this approach and almost certainly in the House of Commons – this should give the UK government the platform to change course," he said.
