Britain’s attorney general has told MPs that the Northern Ireland backstop would be as painful for the European Union as for the United Kingdom, making it likely to be short-lived if it is invoked.

Geoffrey Cox said that although Britain would not be able to withdraw unilaterally from the backstop, the EU would not have an interest in prolonging the arrangement.

“It would be as much an instrument of pain to the European Union as it would be to the United Kingdom,” he said.

“Think of what the European Union is now accepting. It accepts Northern Ireland to have free circulation of its goods not only into the single market but to Great Britain. No other single market trader will have that advantage.

“Hundreds of single market traders throughout the European Union are going to resent the fact that if you are a Northern Ireland business, situated one mile north of the Border, your goods can flow smoothly into the single market and smoothly into Great Britain while theirs can’t.”

Mr Cox was making a statement to the House of Commons after MPs voted to compel the government to publish his legal advice to the cabinet on Brexit. The government published an overview of the advice but Mr Cox said giving MPs full details of what he had told the cabinet would not be in the public interest.

The backstop, which would keep Northern Ireland in closer alignment with the EU than the rest of the UK unless some other solution is found to keep the Border open, is the most controversial element of the withdrawal agreement. The DUP has promised to vote against the Brexit deal, as have dozens of Conservative MPs and all the opposition parties.

‘Irish Sea border’

Mr Cox acknowledged that the backstop would see some regulatory divergence between Northern Ireland and the rest of the UK and that some checks would be needed on goods crossing the Irish Sea.

“While that border would exist, and I find it myself distasteful, nevertheless they are mitigable. And the question again is whether that is a feature that should lead us to decline this deal, which is the best way, I firmly believe, of ensuring that we leave the European Union on March 29th,” he said.

Labour, the SNP, the DUP, the Liberal Democrats and Paid Cymru wrote to the Speaker on Monday calling for proceedings to be launched against the government for contempt of parliament. Labour’s shadow Brexit secretary Keir Starmer said the government’s failure to obey parliament’s instruction to publish Mr Cox’s legal advice gave the opposition choice but to call for such proceedings.

“The government has failed to publish the attorney general’s full and final legal advice to the cabinet, as ordered by parliament. We have therefore been left with no option but to write to the speaker of the House of Commons to ask him to launch proceedings of contempt,” he said.

The publication of the summary on Monday follows earlier comments by the DUP’s Brexit spokesman Sammy Wilson that the full legal advice would expose Ms May as a “liar”.

“The PM clearly doesn’t want the legal advice published because she knows it makes a liar of her,” he told Monday’s News Letter.

“She has been going round saying that the backstop would only be a temporary arrangement, but the legal advice she has makes it clear that is not the case,” he added.

“It proves that she is selling the agreement on a false premise.”

The summary reiterated the Irish backstop would continue to apply “unless and until it is superseded” by an alternative agreement.

Legal advice

The 43-page Legal Position On The Withdrawal Agreement states: “The main provisions of the Protocol [covering the backstop] come into force from the end of the implementation period (December 31st, 2020 — see Article 185 of the Agreement) in the event that a subsequent agreement is not in place by then, and the Protocol will continue to apply unless and until it is superseded, in whole or in part, by a subsequent agreement establishing alternative arrangements (Article 1(4), and the fifth recital in the preamble).”

But if the implementation period is extended then the backstop will not apply until after the end of that period, the paper states.

“However, if the implementation period is extended in accordance with Article 3 of the Protocol and Article 132 of the Agreement, then the Protocol will not start to apply until after the end of that extended period. Certain provisions, relating to preparatory work or the obligation to use best endeavours to conclude an agreement to replace the Protocol, come into force immediately on the coming into force of the Agreement.”

The UK also faces making additional payments to Brussels if the Brexit implementation period is extended, the legal advice says.

Separately, it has been reported that Ms May’s chief Brexit adviser secretly warned her that the backstop agreed in her withdrawal deal with Brussels was a “bad outcome” for Britain.

In a letter to the British prime minister, Oliver Robbins said it would mean the imposition of regulatory checks between the North and the rest of the UK, according to The Daily Telegraph. – Additional reporting PA/Reuters