The advice has been published after MPs found ministers in contempt of parliament
A proposed temporary backstop for the Irish border could last indefinitely, according to Brexit legal advice published by the British government.
The six-page document also says that any route out of the backstop would have to be "political."
The document says the backstop would see Northern Ireland "treated as a third country" for regulatory purposes in terms of goods passing from Great Britain into Northern Ireland.
"This means regulatory checks would have to take place between NI and GB, normally at airports or ports, although the EU now accepts that many of these could be conducted away from the border".
It also says that any Brexit transition period can only have one extension before July 2020.
It reads: "In other words, the withdrawal agreement does not allow for more than one extension".
In his closing remarks, British Attorney-General Geoffrey Cox says: "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.
"This risk must be weighed against the political and economic imperative on both sides to reach an agreement that constitutes a politically stable and permanent basis for their future relationship.
"This is a political decision for the government".
The legal advice was published, after MPs found ministers in contempt of parliament for not publishing it on Tuesday.
The government of British Prime Minister Theresa May was defeated by 18 votes - with 311 MPs backing the motion.
After the vote the Conservative leader in the House of Commons, Andrea Leadsom, announced that the government would publish the full text on Wednesday.
The vote came shortly before MPs began five days of debate on the Brexit withdrawal agreement, ahead of a crucial vote next Tuesday.
Reacting to the advice, deputy leader of Northern Ireland's DUP Nigel Dodds says: "This advice concisely sets out the stark reality of the operation of the backstop.
"Its publication demonstrates how the Prime Minister has failed to abide by the commitments she gave in that the United Kingdom as a whole would leave the European Union and that she would ensure there would be no customs or regulatory divergence within the United Kingdom.
"This backstop is totally unacceptable to Unionists throughout the United Kingdom and it must be defeated and arrangements renegotiated that uphold the commitments which the Prime Minister and her government has in the House of Commons."
The DUP is propping up Mrs May's government at Westminster through a confidence and supply arrangement.
While UK Labour's Brexit Secretary Keir Starmer says the text reveals the "central weaknesses in the government's deal" .
In a tweet, he says it is "unthinkable" why they tried to keep this information from the British parliament.
He writes: "Having reviewed the Attorney General's legal advice, it's obvious why this needed to be placed in the public domain.
"All week we have heard from government ministers that releasing this information. could harm the national interest. Nothing of the sort.
"All this advice reveals is the central weaknesses in the government's deal.
"It is unthinkable that the government tried to keep this information from parliament - and indeed the public - before next week's vote."
All week we have heard from Government ministers that releasing this information. could harm the national interest. Nothing of the sort. All this advice reveals is the central weaknesses in the Government’s deal. 2/3— Keir Starmer (@Keir_Starmer) December 5, 2018