Arlene Foster, the head of Northern Ireland's DUP, which props up the UK's minority government in London, says her party will not support "arrangements that create barriers to trade" between the province and the rest of the country.
The Northern Irish party propping up Britain's minority government will not support any Brexit deal that sees the province operate under different regulations to the rest of the United Kingdom, its leader said on Saturday.
The border between EU-member Ireland and Northern Ireland, which will be the UK's only land frontier with the bloc after its departure, is one of three issues Brussels wants broadly solved over the next 10 days before it decides whether to move the talks onto a second phase about trade.
The Irish government wants Britain to spell out in writing how it intends to make good on its commitment that the 500-kilometre border will remain as seamless post-Brexit as it is today before it will sign off on the first phase of talks.
Dublin and EU officials say the best way to avoid a "hard border" is to keep regulations the same north and south but the pro-Brexit Democratic Unionist Party (DUP), which holds the balance of power in London, said it will not stand for Northern Ireland to operate differently from the rest of the UK.
"We will not support any arrangements that create barriers to trade between Northern Ireland and the rest of the United Kingdom or any suggestion that Northern Ireland, unlike the rest of the UK, will have to mirror European regulations," Arlene Foster said in the text of a speech to be delivered at her party's annual conference.
"The economic reality is that our most important trading relationship is with the rest of the United Kingdom and we will do nothing that puts that at risk in any way."
Foster said she wanted a "sensible Brexit" that recognises the reality of the British province's geography and history, keeps the border open for people to move freely and sees continued trading across the border.
The DUP has so far not presented a solution that would resolve the differences between Britain and the European Union on the border.
British Prime Minister Theresa May said on Friday that London and Dublin would continue to seek to find solutions to achieve their joint aim of ensuring that movement of people and trade across that border can carry on as now.
The role of both Irish nationalist and unionist parties from Northern Ireland in the Brexit talks has been hampered by their failure to restore the province's devolved power-sharing government since its collapse almost a year ago.
Foster said she believed devolution was still the best way to govern the region but her deputy leader, Nigel Dodds, warned that the moment was fast approaching when a return to direct rule from London "will be the lesser of two evils."