The UK would be “indefinitely committed” to EU customs rules if Brexit trade talks broke down, the chief law officer has said.
But Geoffrey Cox said it would not be in either side’s political interests to allow that to happen.
The attorney general said it was a “calculated risk” and “I do not believe we will be trapped in it permanently”.
Labour is leading a cross-party bid to force Mr Cox to publish his legal advice to the government in full.
It came after Mr Cox published an overview of his advice on Brexit, which he described to MPs as a “commentary”.
What is the row over legal advice?
Senior MPs from six parties – Labour, the Lib Dems, the SNP, the Democratic Unionist Party, Plaid Cymru and the Green Party – have written a joint letter to Commons Speaker John Bercow urging him to launch contempt of Parliament proceedings.
MPs voted last month to require the government to lay before Parliament “any legal advice in full”.
The Government has failed to publish the Attorney General’s full and final legal advice on the Brexit deal, as was ordered by Parliament.
We have been left with no option but to write to the Speaker of the House of Commons to ask him to launch proceedings of contempt. pic.twitter.com/LNfUmHg7Tb
— Keir Starmer (@Keir_Starmer) December 3, 2018
Labour’s shadow attorney general Nick Thomas-Symonds accused Mr Cox of “hiding” his full legal advice on Theresa May’s Brexit deal “for fear of the political consequences”.
He accused the government of “playing for time, hoping that the contempt proceedings take longer than the timetable for the meaningful vote”.
Mr Bercow told the MPs he would deliver a verdict on whether they would be granted a debate and a vote on whether the government was in contempt later on Monday or “at the earliest opportunity tomorrow”.
Mr Cox mounted an impassioned defence of his decision not to publish the full advice, insisting it would not be “in the national interest” to break a longstanding convention that law officers’ advice to ministers is confidential.
He said he would answer all MPs’ questions and there was no cover-up, adding: “There is nothing to see here.”
He angrily told MPs calling for it to be published to “grow up and get real”, prompting Green MP Caroline Lucas to accuse him of indulging in “amateur dramatics” and turning the debate into a “farce”.
Nigel Dodds, leader of the Democratic Unionist Party at Westminster, told Mr Cox MPs needed to see the full legal advice he had given to the government – and denounced the Irish “backstop” plan as “deeply unattractive”.
What is the Northern Ireland backstop?
The Northern Ireland “backstop” is a last-resort plan designed to prevent a return to a visible border on the island of Ireland, with both the UK and the EU saying they want to avoid anything which puts the peace process at risk.
It would be triggered if no trade deal has been agreed with the EU that avoids the need for a hard border by the end of the 21-month transition period after 29 March’s official departure date.
It would keep the whole of the UK tied to EU customs rules, unless the two sides come up with another way of avoiding border checks.
Under the EU withdrawal agreement, neither side can unilaterally withdraw from it – but it is meant to be temporary.
Could it become permanent?
Geoffrey Cox told MPs: “We are indefinitely committed to it if it came into force. There is no point in my trying any more than the government trying to disguise that fact.
He said he would have preferred to see a clause inserted into the EU withdrawal agreement that allowed the UK to exit the backstop if negotiations “had irretrievably broken down”.
But he added: “I am prepared to lend my support to this agreement because I do not believe that we are likely to be entrapped in it permanently.”
He said Theresa May’s deal “represents a sensible compromise”.
“It has unattractive elements, unsatisfactory elements for us,” he went on, but MPs had to “weigh it against other risks” such as the “chaos” of a no-deal Brexit or no Brexit.
He urged MPs to “take the key of the cell door and get out on 29 March – and this deal is the best way of doing that”.
MPs will have five days of debate on the proposed Brexit deal, starting on Tuesday. They will decide whether to accept or reject the package in a vote on 11 December.
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.