BRITAIN’S top legal adviser to the European Court of Justice has warned Brexit is endangering the Northern Ireland peace process.

In a forthright intervention, Eleanor Sharpston, UK Advocate General at the Luxembourg institution, argued the decision to leave the bloc had failed to consider its impact on the Good Friday Agreement.

“It is very evident that there is a problem called the border between Northern Ireland and the Republic,” she said during a briefing with journalists from Scotland. “It is rather important for Ireland – and the island of Ireland – that the Good Friday Agreement holds.

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“That agreement was ratified by plebiscites on the two sides of the border on the same day by crushing majorities, by majorities which were way in excess of the margin by which the 2016 referendum ended.

“Although we have had 20 years of the Good Friday Agreement, my senses and my sense of talking to friends in the north, and talking to friends in the south, is that although peace is there its roots are maybe not quite as deep as one would like them to be.”

Sharpston added: “Therefore, whatever the arrangement is for that border, it has to be a border which does not jeopardise what has been built over the last 20 years.”

Theresa May wants the UK to “maintain a common rulebook for all goods” with the EU post-Brexit, including agricultural products.

The Prime Minister has proposed that a treaty should be signed committing the UK to “continued harmonisation” with EU rules – avoiding friction at the UK-EU border, including Northern Ireland.

She wants Parliament to oversee the UK’s trade policy and to have the ability to “choose” to diverge from the EU rules, “recognising that this would have consequences”.

“Co-operative arrangements” would be established between EU and UK competition regulators.

“Different arrangements” would be organised for services “where it is in our interests to have regulatory flexibility”. Under the Prime Minister’s plan a “joint institutional framework” would be established to interpret UK-EU agreements. This would be done in the UK by UK courts, and in the EU by EU courts.

However, decisions by UK courts would involve “due regard paid to EU case law in areas where the UK continued to apply a common rulebook”.

Cases will still be referred to the European Court of Justice as the interpreter of EU rules, but it “cannot resolve disputes between the two”.

In a withering attack, Sharpston criticised the Prime Minister’s plans for seeking a common rule book on goods, while wanting to take the UK out of the jurisdiction of the European Court of Justice. She argued the two aims are incompatible and the UK could not have a separate court to oversee and rule on its system of standards.

The lawyer insisted a common rule book on goods is required to be interpreted exactly same way on both sides of the Irish border, meaning that the UK would have to come under the jurisdiction of the court.

“If you want to have frictionless trade it is necessary that a common rule book be interpreted in the same way on both sides of the invisible, therefore frictionless, border,” Sharpston said.

“As soon as you get divergent interpretations, you can no longer have frictionless trade. That’s a very simplistic and, I hope, actually rather an uncontroversial statement as a lawyer. What one does as a politician with that statement and that reality, I do not know.

Sharpston also expressed frustrations over some Conservative ministers’ understanding of European law.

“I spent a certain amount of my practising life at the bar explaining to national judges how EU law operated. They were on the whole very courteous, listened to me patiently, went away and read and grappled with the concepts,” she said.

“I do have a concern that the legal issues are rather complicated and that the amount of time available to understand them may not be as generous as it needs to be.”