Britain’s top government lawyer warned ministers of “a legal risk” in the Brexit deal, according to advice which the government was forced to publish on Wednesday.
The six-page document focuses on a legal guarantee in the deal to keep the Irish border open by giving Northern Ireland a different economic status from mainland Britain.
The so-called backstop would also leave Britain as a whole in an effective customs union with the EU to keep the border between Ireland and Northern Ireland free-flowing.
In the document, Attorney General Geoffrey Cox told ministers that if it enters into the arrangement, Britain could stay in “indefinitely” if no other solution is found.
“Despite statements in the Protocol that it is not intended to be permanent… in international law the Protocol would endure indefinitely until a superseding agreement took its place,” the text said.
“In the absence of a right of termination, there is a legal risk that the United Kingdom might become subject to protracted and repeating rounds of negotiations,” it said, adding that these could last “many years”.
The advice spelled out that in terms of trade mainland Britain would be “essentially treated as a third country” by Northern Ireland for goods passing between the two.
Mainland Britain would also have to levy the EU’s common external tariff for any goods coming from non-EU countries.
The backstop arrangements have incensed Northern Ireland’s Democratic Unionist Party which has propped up the government and wants no trade border with the rest of Britain.
But the advice also said that the backstop was “by no means a comfortable resting place in law for the EU” since it disrupted the EU’s rules on free movement of people, goods, capital and services.
“It is arguable that this poses a dangerous precedent for other member states at risk of seeking to exit,” Cox wrote.
He said it created a “difficult precedent” for the EU because of other member states with overseas territories who might want similar provisions.
The government resisted weeks of pressure from the main opposition Labour Party to publish the legal advice but was forced to do so when a contempt motion against it was passed by parliament on Tuesday.