Mr Cox wrote: "However, 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.no internationally lawful means of exiting the Protocol's arrangements, save by agreement".
May said it is time for British lawmakers to come together to back what she called an improved Brexit deal and to fullfil their promise to the British people. Can the public ignore this vote as Parliament has ignored the will of the.
The so-called backstop that is part of the UK's withdrawal agreement from the European Union is an insurance policy created to ensure there is no hard border - no customs checks or other border structures - after Britain breaks away.
However, the text of the 585-page withdrawal agreement remained unchanged.
The PM said: "What we have secured is very clearly that the backstop can not be indefinite".More news: Sephora urged to drop partnership with Lori Loughlin's daughter Olivia Jade
Juncker believed "it is high time to complete the withdrawal process in line with the wishes expressed by the government of the United Kingdom and to move on as swiftly as possible to the negotiation of our future partnership".
Mr Juncker warned that if MPs voted down the deal a second time, "there will be no third chance". It is what we do with the second chance that counts. "There will be no further interpretations of the interpretations, no further assurances of the reassurances if the "meaningful vote" tomorrow fails".
"Let us be crystal clear about the choice - it is this deal or Brexit might not happen at all".
But a legal opinion commissioned from three senior lawyers by the People's Vote campaign for a second referendum said there was "no basis" for Mr Cox to alter his advice on the indefinite nature of the backstop.
Meanwhile, the view in Dublin is that the original Withdrawal Agreement has not changed legally, with Leo Varadkar using the word "complementary" to describe the changes made between May and European Union negotiators last night.
"It's important to see the details, but the DUP's support would be a very important and significant factor".More news: Callum Hudson-Odoi reveals Chelsea FC’s best player in training
The main sticking point is the so-called Irish border backstop, an insurance policy aimed at avoiding controls on the border between the British province of Northern Ireland and EU-member Ireland after Brexit.
Theresa May revealed last night that she had obtained new "legally binding assurances" (a phrase that legal experts have said is not in any way factual terminology) from European Union negotiators ahead of her vote in the Commons this evening on a Brexit Withdrawal Agreement.
Labour leader Jeremy Corbyn has called on MPs to reject the deal and accused Mrs May of a plan to "recklessly run down the clock" before March 29.
In a personal blow to the embattled leader, MPs rejected her deal by 391 votes to 242 after 24 hours of frantic diplomacy and a wildly fluctuating pound.
A motion in Mrs May's name, authorising the Prime Minister to request an extension to the two-year Article 50 negotiation process, was passed by 413 votes to 202 - a majority of 211.
In a letter to EU Council president Donald Tusk, Juncker left the option of an extension to Article 50, but only by a could of months.More news: Lori Loughlin Has Been Dropped By Hallmark Over Admissions Scheme
Juncker on Monday said a delay beyond European Parliament elections at the end of May would mean Britain would have to take part in the polls.