Mr Cox wrote: "I now consider that the legally binding provisions of the Joint Instrument and the content of the Unilateral Declaration reduce the risk that the United Kingdom could be indefinitely and involuntarily detained within the Protocol's provisions at least in so far as that situation had been brought about by the bad faith or want of best endeavours of the EU".
On the eve of a crucial vote in the House of Commons on Prime Minister Theresa May's divorce deal, Michel Barnier said the British leader must negotiate with MPs rather than the EU. Afterward, hard-core Brexit supporters in May's Conservative Party and the prime minister's allies in Northern Ireland's Democratic Unionist Party both said they could not support the deal.
They said its potentially indefinite nature and the UK's inability to withdraw from it unilaterally eroded the UK's sovereignty and did not deliver a true withdrawal from the EU.
DUP leader Arlene Foster told the BBC she was sympathetic to demands for a day's delay to give time to study the assurances.
"And more and more people, who previously voted for "Remain", are now supporting "Leave", he said.
She concluded the meeting by telling ministerial colleagues: "Today is the day. Maybe I shouldn't let you do it, I'll just get you in trouble".
UK AG G.Cox said the legal risk of Brexit deal is intact. "We do not think today is the right time to test the will of the House on the case for a new public vote", the People's Vote campaign said in a statement.
"Long suspected, now confirmed", said Chris Leslie, one of TIG's former Labour MPs.
There were now "material new obligations" on the European Union to pursue alternative arrangements and it would be "unconscionable" if the European Union refused to "consider or adopt reasonable proposals relating to alternative arrangements". "The Government's strategy is now in tatters".
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Mr Cox's advice was released as Mrs May's agreement was being scrutinised by the so-called "Star Chamber" of Brexiteer lawyers convened by the European Research Group (ERG) of Leave-backing Tory MPs.
"Since her Brexit deal was so overwhelmingly rejected, the prime minister has recklessly run down the clock, failed to effectively negotiate with the European Union and refused to find common ground for a deal parliament could support".
May's deputy, David Lidington, then announced to MPs that the United Kingdom had secured three changes to the agreement that May had put before the Commons last Monday.
The votes came as US President Donald Trump said Brexit was ripping Britain apart and warned that another referendum would be "unfair".
After two-and-a-half years of haggling since the 2016 Brexit referendum, Juncker cautioned this was the last chance for Britain.
If we end up staying beyond a certain period, we have to take part in the European elections.
Opposition to May's deal among members of the Conservative Party derives from a belief that it does not offer the clean break from the European Union that many voted for.
"I would hope it will be put to bed and we can all face up to the reality of the position and the opportunity that we have", Mr Johnson told MPs before the vote.
Dutch Prime Minister Mark Rutte tweeted that he was "pleased with the agreement" and implored that British legislators approve the deal.
One on Wednesday would decide whether Britain should simply leave on March 29 without any deal at all.
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.