"The legal risk remains unchanged that if through no such demonstrable failure of either party, but simply because of intractable differences, that situation does arise, the United Kingdom would have, at least while the fundamental circumstances remained the same, no internationally lawful means of exiting the Protocol’s arrangements, save by agreement", Cox said in a letter to May published by the UK government.
Another document lays out commitments to developing new technology at the border, while the third one says that the United Kingdom might be able to "disapply" backstop if the European Union did not act in good faith.
READ MORE: May, Juncker Agree on Changes to Brexit Deal Ahead of Commons Vote
Juncker stressed at a late-night press conference on Monday that there would be no third chance if UK lawmakers voted against the tweaked deal on Tuesday.
The United Kingdom is set to leave the European Union on March 29, but the withdrawal agreement has already been rejected by the UK House of Commons once, before reassurances were added.