The UK will not be able to “unilaterally” quit the EU’s customs rules under the Irish backstop, the UK’s chief law officer has said.
But Geoffrey Cox said the so-called backstop was meant to be temporary arrangement and it was a “political question” as to when it would end.
It comes after Mr Cox published an overview of his legal advice on Brexit.
Ministers are under pressure to publish the full advice, with Labour warning of a “constitutional crisis”.
The prime minister says the advice is confidential, but some MPs think ministers do not want to admit it says the UK could be indefinitely tied to EU customs rules.
In a Commons statement, Mr Cox said he “strongly believed” the Commons would pass the Brexit deal – but it was a “political decision that each one of us must make” not a legal one.
Sam Gyimah, who quit the government on Friday, said releasing the full advice was “key to restoring trust in politics”.
MPs voted last month to require the government to lay before Parliament “any legal advice in full”.
His statement to the House of Commons will be followed by five days of debate on the proposed Brexit deal, which will get under way on Tuesday.
The UK is due to leave the European Union on 29 March, 2019. The UK and the EU have agreed a detailed plan for how Brexit happens, with a draft outline of how they want the future relationship to work.
But the plan has to be backed by both the UK and EU parliaments before 29 March, if the UK is to leave under these terms. If it is not ratified then the UK would be on course to leave the EU without any deal – although a delay to Brexit to allow either a renegotiated deal or a further referendum have been floated as alternatives.
Legal issues to the fore
According to the Sunday Times, Mr Cox told ministers in a letter the only way out of the “backstop” agreement – an arrangement that will come into force unless another way is found of avoiding a return to a visible Northern Ireland border – would be to sign a new trade deal, a process which could take years.
Brexit-supporting MPs say it could mean an open-ended commitment for the UK, forcing it to remain in the EU’s customs union while details of the deal are being worked out.
Labour is planning to join forces with other parties, including the DUP, which keeps Mrs May in power, to initiate contempt of Parliament proceedings unless the government backs down on publishing the full legal advice.
The former Lord Chancellor Lord Falconer told BBC Radio 4’s Today programme that the public “have the full right to know” the legal advice and the debate in the Commons should “properly be informed” by it.
And Mr Gyimah, who until Friday was a science minister, said the government had a duty to “level with the public”.
But former solicitor general and Tory peer Lord Garnier told the programme it was a “matter of convention” that the advice was not disclosed, adding that it was government policy under attack and not the legal advice.
What’s in the legal overview?
The Attorney General’s position paper contains little that we did not know already, but it does set out in black and white the legal ramifications of the EU withdrawal agreement, which guarantees citizens rights, the £39bn “divorce bill” and the 21-month transition period.
It confirms that the withdrawal agreement “does not contain any provision on its termination”.
“In the absence of such a provision, it is not possible under international law for a party to withdraw from the Agreement unilaterally.”
It also confirms that the UK will be excluded from EU decision making during the transition period after 29 March’s official departure date – and that it may be excluded “from having access to ‘security related sensitive information’ in certain exceptional circumstances”.
And it says the Brexit deal does not breach Northern Ireland’s Good Friday peace agreement.
The paper also sets out details of how the Northern Irish backstop will work.
Once triggered, the backstop will continue to apply “unless and until its provisions are superseded by a subsequent agreement between the UK and the EU”.
But, the paper adds, it is the “intention” of both sides that “if it does start to apply then it should do so only temporarily”.
How did we end up with the backstop?
The prime minister’s chief Brexit adviser, Olly Robbins, told MPs that the backstop was “not the future relationship that the UK and EU want to have with each other” but, without it, there would be no wider exit agreement.
“It is a necessity and a somewhat uncomfortable necessity for both sides,” he told the Commons Brexit committee.
Asked if the government had drafted a clause for the Withdrawal Agreement which would have allowed the UK to opt out of the backstop unilaterally, Mr Robbins said: “Ministers asked us to look at a whole range of options for how to bring the backstop to an end, and so we did.
“And the prime minister and other ministers tested some of those out on European partners.
“But, what we went into the negotiation with in the end was a text that delivered the termination clause very much as it is laid out there.”
Boris Johnson, who resigned from government over the PM’s Brexit vision, described the arrangement in his weekly Daily Telegraph column as “a great steel trap that is about to clamp its jaws around our hind limbs and prevent our escape”.
The DUP’s Sammy Wilson said it was important for MPs to know exactly what they are voting for and the implications for Northern Ireland.
Ministers vs MPs
Speaking on ITV’s This Morning, Theresa May urged MPs to “hold their nerve” and get Brexit “over the line” – suggesting Parliament had a “duty to do what people asked us to do” by taking the UK out of the EU.
“This is not any old vote, this is delivering on what people voted for in the referendum and it is important for us as politicians to remember that,” she said.
Nigel Evans, one of those Tory MPs to say they will vote against the deal, told Sky News the prime minister was “in a hole of her own digging and she can either get out of that hole or call for the JCBs”.
He said he had supported Mrs May after her “awful” party conference speech and “appalling” election campaign last year but now “she needs to help herself a bit” by changing course.
Ministers insist they can persuade MPs currently against the deal to change their mind because, they claim, the alternative is a no-deal Brexit or “no Brexit at all”.
Home Secretary Sajid Javid told BBC Radio 4’s Today the agreement on the terms of the UK’s exit and its future relations with the EU was “never going to be perfect” but it was the “right deal for the country”.
He insisted the agreement delivered on the 2016 referendum result by bringing “a complete end” to EU free movement in the UK – although he conceded it was “very unlikely” MPs would see the details of what the UK’s post-Brexit immigration system might look like before they vote, as the plans were still be worked through.
Meanwhile, as campaigners continue to push for a further referendum, an e-petition urging the government to rule that option out, will be debated by MPs from 16:30 GMT.