The advice says that discussions on any extension would involve "reaching further agreement on the UK's financial contribution".
The letter was sent to the Speaker of the UK Parliament's lower chamber House of Commons, who chairs debates in the chamber and must now decide on whether to allow a contempt motion to be voted on by the house.
May is battling to persuade lawmakers to support the divorce agreement between Britain and the European Union when the House of Commons votes on December 11.
The dense legalese paper states that the backstop "protocol", which would see the whole of the United Kingdom enter into a customs arrangement with the European Union, 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that a subsequent agreement is not in place by then, and the protocol will continue to apply unless and until it is superseded, in whole in or part, by a subsequent agreement establishing alternative arrangements".
She will say that she had meetings with leaders keen to strike trade deals, including the hosts Argentina, Australia, Chile and Japan.
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AIDS remains the leading cause of death in women aged 15 to 44, and the second-largest mortality factor for teenagers worldwide. The group and other NGOs send volunteers into the field to survey people and convince them to get tested.
"I make no bones about it - I would have preferred to have seen a unilateral right of termination in this backstop", he told MPs. "They are essentially either we leave with no deal which would have serious economic consequences, or we say to the British public: 'I'm sorry you got it wrong, you're going to have to do it again, ' which I think would have serious democratic consequences". Combined with Labour, the Liberal Democrats and opposition from the DUP - which is in a confidence-and-supply agreement with May's government - the bloc would likely be powerful enough to defeat the motion, and reports have suggested the government may even consider cancelling the vote entirely.
Opposition parties including Theresa May's DUP allies said the document and the Attorney General's statement failed to meet the terms of a vote demanding publication of the full legal advice and asked Speaker John Bercow to launch contempt of Parliament proceedings.
"I am caught in an acute clash of constitutional principle", he said.
"So, it is a necessity and a slightly uncomfortable necessity for both sides". The backstop will last indefinitely, until it is superseded by the treaty setting out our future relationship, unless the European Union allows us to exit.
"And the Prime Minister and other ministers tested some of those out on European partners. We will have very little leverage".
The UK faces making additional payments to Brussels if the Brexit implementation period is extended, the Government's Brexit legal advice also said.
Under the Fixed-term Parliaments Act 2011, the government in theory has a five-year term, but a vote of no confidence or a vote by two thirds of the Commons can still trigger a general election at any time, making it exceedingly hard for the government of the day to stave off an election should lawmakers wish to go to the voters.