An Irish border backstop installed in the case of a no-deal Brexit would continue to apply “unless and until it is superseded” by a new agreement, Government legal advice says.
The position statement released on Monday afternoon also said Britain faced paying extra money to the European Union if the implementation period after the UK leaves in March has to be extended.
The paper also says the UK would not be able to back out of Theresa May's Withdrawal Agreement without the agreement of the EU if it is passed by MPs.
The Legal Position On The Withdrawal Agreement was published after the Government lost a Parliamentary vote calling for the full legal advice to be released.
However, the full text has not been released and Labour had said it would join with opposition parties and initiate contempt of Parliament proceedings in a bid to force the Government's hand.
Mr May earlier insisted she would still have a job in two weeks’ time as she faces a crunch December 11 Commons vote on her EU Withdrawal Agreement.
The dense legalese paper states that the backstop “protocol”, which would see the whole of the UK enter into a customs arrangement with the EU, would come into effect in December 2020 “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that a subsequent agreement is not in place by then, and the protocol will continue to apply unless and until it is superseded, in whole in or part, by a subsequent agreement establishing alternative arrangements”.
However, it goes on to say that “if it does start to apply then it should do so only temporarily”.
The UK faces making additional payments to Brussels if the Brexit implementation period is extended, the legal advice also says.
Under the terms of the Withdrawal Agreement, it is due to run until the end of December 2020 but can be extended by up to two years if both sides agree.
The advice says that discussions on any extension would involve “reaching further agreement on the UK’s financial contribution”.
The Government’s chief legal officer, Attorney General Geoffrey Cox, was due to address MPs on Monday afternoon.
Earlier Mrs May’s chief Brexit adviser told MPs that the Northern Ireland border backstop was a “slightly uncomfortable necessity” for both the UK and the European Union.
The fallback plan agreed with Brussels was “not the future relationship that either the UK or the EU wants to have with one another”, Olly Robbins told the Exiting the European Union Committee.
He said: “It is an uncomfortable position for both sides and the reality...is that there is not a withdrawal agreement without a backstop.
“That reflects also, as I’ve said to this committee before, ministers’ commitments to Northern Ireland and to avoid a hard border on the island of Ireland, rather than being something imposed upon us.
“So, it is a necessity and a slightly uncomfortable necessity for both sides.”