The UK could be stuck in “repeated rounds of negotiations” to leave the EU for years with no lawful power to exit, according to the government’s legal advice.
A six-page document analysing the implications of Theresa May’s deal warned the mechanism to prevent a hard border on the island of Ireland returning could remain indefinitely.
It was written by Attorney General Geoffrey Cox and sent to Theresa May on 13 November, but only published on Wednesday after wrangling between MPs and the government.
MPs had been left reeling by news they would only be given a 43-page summary document – and subsequently found the government in contempt of parliament for trying to keep the full advice out of the public domain.
Following a trio of defeats in the Commons on Tuesday, the document was released minutes after Prime Minister’s Questions began.
Theresa May was swiftly tackled by the SNP’s Westminster leader Ian Blackford, who accused of “concealing the facts on her Brexit deal”.
Mrs May rejected the claim, insisting the document contained the same information as a shortened statement by Mr Cox earlier this week.
But the assurances failed to appease her government partners, with the Democratic Unionist Party’s deputy leader Nigel Dodds describing the revelations for Northern Ireland as “devastating”.
He said the legal advice made it clear that the proposed backstop arrangement was “unacceptable” and must be defeated.
Shadow Brexit secretary Sir Keir Starmer also said that full legal advice, which runs to 33 paragraphs, revealed “the central weaknesses in the government’s deal”.
In the document, Mr Cox wrote that the backstop protocol “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”.
He said that would remain the case “even if parties are still negotiating many years later and even if the parties believe that talks have clearly broken down and there is no prospect of a future relationship agreement”.
And in a segment that caused the most fear among MPs, the attorney general warned: “The [backstop] would endure indefinitely until a superseding agreement took its place in whole or in part.”
Mr Cox also warned ministers that Britain would essentially become a third country when importing goods to Northern Ireland and would become subject to additional border checks.
Tom Brake, the Liberal Democrats’ spokesperson for Brexit, said such extra checks would have a huge impact on British business.
He told Sky News: “We are talking about British businesses having to complete customs checks for goods going to Northern Ireland.
“Brexit now comes with a huge impact, possibly of the break up of the United Kingdom. These arguments are now being made with rather more evidence than before the vote.”
Andrea Leadsom, leader of the House of Commons, said the government had previously refused to publish the full legal advice on the Brexit deal as a “point of principle”.
She told Sky News: “There are conflicting constitutional principles here. Of course the House of Commons has the right… to require papers.
“But there’s a really important constitutional principle whereby law officers give confidential, frank advice to government ministers.
“It’s very existence isn’t normally published, let alone the advice itself.
“Now, following the vote [on Tuesday]… the problem in future will be law officers will think twice about the type of advice they give to government.
“And the government will have to think very carefully about what sort of advice they ask for, for fear that the House of Commons might again in future ask for it to be fully disclosed.”
Despite concerns about the ramifications of releasing the full legal advice, Mr Cox described his decision to make it available to MPs as “exceptional” – and said it did not set a precedent for the future.
He said the use of the arcane parliamentary procedure to force the release of the document created “constitutional tensions” which “are not themselves conducive to the proper conduct of public affairs”.
The future of the DUP’s alliance with the government does appear more clear.
The pro-Brexit European Research Group of backbench Tory MPs met on Wednesday evening – and a source in the room said they were told the DUP will drop support for Mrs May if her Brexit deal gets through the Commons on 11 December.
However, if the deal fails, the party plan to back her government in a confidence vote.
This would scupper Labour’s chances of forcing a general election if the deal does sink next week – as it is expected to.
MPs spent the rest of Wednesday discussing the divorce deal – with three more days of debate before a so-called “meaningful vote” on 11 December.