The legal risk of the UK being tied to EU rules after Brexit “remains unchanged” despite changes to the PM’s deal, the attorney general has said.
Geoffrey Cox said the new agreements reinforced the legal rights available to the UK if post-Brexit trade talks broke down due to “bad faith”.
But the Brexit-backing European Research Group have said they will not be voting for the PM’s deal.
That is a blow to Mrs May’s hopes of getting it through the Commons later.
In a statement, the ERG said: “In the light of our own legal analysis and others we do not recommend accepting the government’s motion today.”
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In his advice, Mr Cox said the extra assurances won by Mrs May in 11th hour talks with the EU “reduce the risk that the United Kingdom could be indefinitely and involuntarily detained” in the backstop if talks on the two sides future relationship broke down due to “bad faith” by the EU.
But he warned that the question of whether a satisfactory post-Brexit deal on a permanent trading relationship can be reached remained “a political judgment”.
And he said “the legal risk remains unchanged” that if no such agreement can be reached due to “intractable differences”, the UK would have “no internationally lawful means” of leaving the backstop without EU agreement.
In a statement to the Commons, Mr Cox later said: “Were such a situation to occur, let me make it clear, the legal risk as I set it out in my letter of November 13 remains unchanged.”
Reaction from MPs
The last time Mrs May’s withdrawal agreement was put to Parliament in January, it was voted down by a margin of 230.
The BBC’s political editor Laura Kuenssberg said it would be a “political miracle of historic proportions” if Mrs May could overturn such a heavy defeat.
Mrs May earlier addressed a meeting of Conservative MPs, in an effort to change the minds of those opposed to her deal.
Conservative MPs leaving the meeting suggested half of those who voted against deal last time will switch to support it later, said BBC’s Chief Political Correspondent Vicky Young.
Former minister Grant Sharps said the vote would be close and Mrs May “needed the DUP” to back her deal.
Work and Pensions Secretary Amber Rudd said she believed the prime minister’s deal would go through “otherwise instability will follow which would be so unwelcome”.
But Mark Francois, a member of the Brexit-backing European Research Group, said he was “wholly unconvinced” by Mrs May’s improved deal.
And ERG chairman Jacob Rees-Mogg said: “The unilateral declaration doesn’t add anything…