Brexit: Release of full legal advice sparks political row
Theresa May's Brexit backstop plan risks a "stalemate" and "protracted rounds of negotiations" with the EU, the full legal advice on her deal says.
Newly published documents show the PM was told an arrangement designed to prevent a hard Irish border could last "indefinitely" and the UK could not "lawfully exit" without EU agreement.
The Democratic Unionists said this would be "devastating" for the UK.
But Mrs May rejected SNP claims she has misled Parliament on the issue.
Ministers were forced to publish Attorney General Geoffrey Cox's full advice after ministers were found in contempt of Parliament on Tuesday for providing only a legal overview earlier this week.
Speaking in the Commons, the prime minister said there was "no difference" between the two documents and the legal position on her proposed "temporary" customs arrangement with the EU was clear.
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While the UK would have no unilateral right to withdraw from the backstop - a measure designed to prevent the return of physical checks on the Irish border by the UK and EU sharing a single customs territory - she insisted neither the UK nor the EU wanted it to come into force in the first place.
But the Democratic Unionists said they could not accept Northern Ireland being subject to different regulation and judicial oversight from the rest of the UK and effectively becoming a "third country" from Britain.
What does the full advice to PM say?
The government argued that Mr Cox's analysis of the Brexit deal, published on Monday, was adequate and disclosing his full and final advice would be against the national interest.
Labour and other opposition parties said ministers had "wilfully" refused to comply with a binding vote in the Commons last month which demanded full disclosure and MPs agreed in a vote on Tuesday.
In the six-page letter published on Tuesday, Mr Cox said the "current drafting" of the backstop "does not allow for a mechanism that is likely to enable the UK to lawfully exit the UK wide customs union without a subsequent agreement".
"Despite statements that it is not intended to be permanent… in international law the protocol would endure indefinitely until a superseding agreement took its place," he wrote.
"In the absence of a right of termination, there is a legal risk that the UK might become subject to protracted and repeated rounds of negotiations...The resolution of such a stalemate would have to be political."
The backstop has become a defining issue for many Tory critics of the PM's deal - who say that it infringes the UK's sovereignty and will prevent the UK from being able to negotiate its own trade deals.
Mr Cox has warned the backstop is a "calculated risk" and entering into is would be a political decision.
"This risk must be weighed against the political and economic imperative of both sides to reach an agreement that constitutes a politically stable and permanent basis for their future relationship," he wrote.
Analysis: In black and white
The BBC's legal correspondent Clive Coleman
When lawyers give legal advice they are expected to speak frankly.
Many will conclude this advice is franker and starker than the way in which the government has presented the legal implications of the Withdraw Agreement.
In particular on the Northern Ireland backstop, it is there in black and white, that in the absence of an agreement replacing it, the backstop will continue indefinitely. The UK could not force the EU to conclude an agreement bringing it to an end.
That punctures the government's optimism on the issue. Whereas Article 50 allowed the UK to pull out of the EU, there is no provision for the UK to pull out of the withdrawal agreement. That will pour petrol on the flames of the political debate.
May defends advice at Prime Minister's Questions
The prime minister has insisted there can be no Brexit deal without a backstop of some kind to protect commitments made to Northern Ireland in the Good Friday Agreement.
But she came under fire at Prime Minister's Questions from the SNP's Westminster leader Ian Blackford.
"The legal advice is clear," he said. "It states: despite statements in the protocol it is not permanent, in international law the protocol would endure indefinitely. Since the prime minister returned from Brussels with her deal, the Prime Minister has been misleading the House inadvertently or otherwise."
"Is it time that the prime minister took responsibility for concealing the facts on her Brexit deal from members in this House and the public?
The PM told MPs there was "no difference" between the 50-page legal overview published on Monday and Mr Cox's letter from November published on Wednesday.
She also said she had made it clear that the UK could not unilaterally leave the backstop plan, so had not been at odds with the legal advice, which made it clear the UK had no sovereign right to withdraw.
But she insisted that it gave the UK a number of competitive advantages and therefore the EU would not want the UK "to be in it any longer than necessary".
But the DUP's Westminster leader Nigel Dodds said it was now apparent why the government had tried to "hide" the legal position.
For all the Prime Minister’s promises and pledges the legal advice is crystal clear.
In her words, no British Prime Minister could ever accept such a situation...
End of Twitter post by @NigelDoddsDUP
Labour said the advice showed the "central weakness" of the PM's Brexit deal while the Lib Dems said it would leave the UK trapped in a "Brexit hamster wheel".
Debate on Brexit deal continues
The second of five days of debate on Mrs May's Brexit agreement is under way, focusing on security.
The government suffered three embarrassing defeats on Tuesday, on the role that MPs will play if Mrs May's deal in rejected as well as the disclosure of the legal advice.
International Trade Secretary Liam Fox has warned there is a risk of Parliament trying to "steal Brexit from the British people" if the PM's agreement is rejected.
He said there was a "natural Remain majority" in Parliament and that any attempts to delay the UK's departure or overturn the 2016 referendum result would be a "democratic affront".
But former attorney general Dominic Grieve said claims Parliament was trying to block Brexit were a "little far-fetched" and MPs like him were merely concerned with averting a national crisis if Parliament was deadlocked.
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