DUP MP Nigel Dodds says the publication of the full legal advice provided to the British cabinet on Theresa May’s Brexit deal "sets out the stark reality of the operation of the backstop".
He described the details provided in the document as "devastating" and said it made clear the proposed backstop arrangement for the Irish border was "unacceptable" and must be defeated.
The legal advice has warned it could result in the UK becoming stuck for many years in "protracted and repeating rounds of negotiations" with no lawful power to exit.
And it made clear that Brussels could apply to an arbitration panel for Northern Ireland to remain in the EU customs area while the rest of the UK left.
The six-page document by Attorney General Geoffrey Cox was released to MPs a day after the House of Commons found the government in contempt of parliament for trying to keep it secret.
The letter, dated November 13, emerged minutes before Theresa May appeared at Prime Minister's Questions.
Mr Dodds said: "It is clear from the Attorney General's advice on the legal effect of the protocol on Northern Ireland to the Prime Minister and her cabinet colleagues that we were right to advocate its full publication and we have been vindicated in our opposition to the backstop arrangements contained within the Withdrawal Agreement.
"This advice concisely sets out the stark reality of the operation of the backstop.
"Its publication demonstrates how the Prime Minister has failed to abide by the commitments she gave in that the United Kingdom as a whole would leave the European Union and that she would ensure there would be no customs or regulatory divergence within the United Kingdom."
He said the "backstop is totally unacceptable to unionists"... and "must be defeated and arrangements renegotiated that uphold the commitments which the Prime Minister and her government has made".
UUP leader Robin Swann also said Mrs May had "gone against her own word" adding that if the withdrawal agreement was "allowed to proceed it would see Northern Ireland cast adrift".
Mr Cox found that the protocol setting out the terms of the backstop "does not provide for a mechanism that is likely to enable the UK lawfully to exit the UK-wide customs union without a subsequent agreement".
Under the arrangements, "for regulatory purposes, GB is essentially treated as a third country by NI for goods passing from GB into NI", he said.
He said that - despite assurances from London and Brussels that it is intended to be temporary - the protocol would "endure indefinitely" under international law until another agreement takes its place.
The UK would have no legal means of compelling the EU to conclude any such agreement.
The Attorney General warned: "In the absence of a right of termination, there is a legal risk that the United Kingdom might become subject to protracted and repeating rounds of negotiations.
"This risk must be weighed against the political and economic imperative on both sides to reach an agreement that constitutes a politically stable and permanent basis for their future relationship.
"This is a political decision for the government."
Mr Cox's advice stated that, if Brussels felt that negotiations on a trade deal had broken down or were taking too long, it would be able to apply to the arbitration panel for Britain to be removed from the customs union while Northern Ireland remains - effectively creating the border in the Irish Sea, which Mrs May has said no prime minister could accept.
He also noted the backstop arrangement would be "enormously complex" for the EU, requiring "considerable resources", meaning Brussels would come under pressure - especially from Dublin - to bring it to an end.
Meanwhile, a group of young people will travel to Stormont tomorrow to voice their opinions about the Brexit debate.
The activists, who are part of Our Future, Our Choice NI, will travel on the People's Vote bus to Stormont where a rally will be held at 2pm to "show politicians that young people are willing to take action".
