Democratic Unionist Party (DUP) MP Sir Jeffrey Donaldson has said a draft Brexit deal between the UK and Brussels “fundamentally undermines the constitutional and economic integrity of the UK”.
He said his party wants to see Brexit, but “this is not the right Brexit”.
His comments come as talks enter crunch time, with the Irish border the sticking point.
Irish prime minister Leo Varadkar is to hold a special cabinet meeting later.
The meeting to discuss the draft text will take place in Dublin.
DUP leader Arlene Foster is travelling to London on Wednesday morning but the party has said there is no meeting planned with the prime minister.
The draft withdrawal agreement, which has been drawn up alongside a statement of future relations with the EU, has not been published but is thought to run to some 500 pages.
It addresses the Northern Ireland “backstop”, which aims to guarantee that physical checks will not be reintroduced at the border with the Irish Republic, in the event of the EU and UK failing to agree a deal on future trading relations.
The backstop within the agreed draft is believed to avoid a return to a “hard border” with the Republic by keeping the UK as a whole aligned with the EU customs union for a limited time.
The UK government faces a battle to win Parliament’s backing for the draft text and relies on the DUP’s support to win key Commons votes.
The DUP has said there is no planned meeting with the prime minister set for Wednesday.
Sir Jeffrey told BBC Radio Four’s Today programme that the “Northern Ireland backstop has been rolled into a UK-wide backstop” and this could have consequences for the union.
“I think it has been described by one commentator as like a swimming pool, and Northern Ireland will be in at the deep end,” he added.
‘Break-up of the UK’
“If the United Kingdom wants to leave, we will have much closer regulation in terms of both the customs arrangements and the single market, so we will be treated very differently from the rest of the United Kingdom.
“Many people conclude that this will lead to a regulatory border in the Irish Sea. It means the end of the UK single market.”
“In the longer term, when the UK does new trade deals with the rest of the world, Northern Ireland won’t benefit from that, won’t be part of that, because we will have to remain aligned with the European Union and that will lead to the break-up of the United Kingdom,” he added.
The draft deal comes as four Stormont parties that campaigned to remain in the EU are to meet Taoiseach (Irish prime minister) Leo Varadkar in Dublin on Thursday.
The four parties were originally due in Dublin to talk to Mr Varadkar on Wednesday afternoon, but it emerged on Tuesday night that their meeting had been pushed back.
SDLP deputy leader Nichola Mallon has accused the DUP of “hysteria” over the draft text.
She said it was a “good time” for them to revisit the details of the confidence-and-supply agreement the party has with the government,
Irish government in ‘constant communication’
Both the UK and EU have said they want to ensure physical checks will not be reintroduced.
They had each come up with their own proposals for the Northern Ireland backstop, but had been unable to agree on a way forward.
The backstop is the “insurance policy” arrangement to keep trade across the border as frictionless after Brexit as it is now, if no other solution can be found as part of a wider deal with the EU.
UK Prime Minister Theresa May will also hold a special meeting of her cabinet on Wednesday afternoon at 14:00 GMT to set out the details of the proposed text.
Irish national broadcaster RTÉ had reported a source said the Irish government wanted to give the UK cabinet “time and space” to analyse the text before making any detailed response.
A spokesperson for Irish Foreign Minister Simon Coveney told BBC News NI that the government was not commenting on media speculation.
“Michel Barnier and the (EU) taskforce are charged with negotiating with the UK and we are in constant communication with them throughout,” they added.
The Irish government has repeatedly said a backstop cannot have an arbitrary end point, and must apply unless and until some other political or technical development means it is no longer needed.
But pro-Brexit politicians are concerned that a backstop without a time limit or a clear exit route would lock the UK into the EU’s customs union indefinitely.