THERESA May's last-minute Brexit agreements "reduce the risk" that the UK could be trapped indefinitely in the backstop, but do not remove it altogether, Attorney General Geoffrey Cox has said.

The Attorney's legal advice deals a significant blow to the Prime Minister's hopes of securing MPs' backing for her EU Withdrawal Agreement in the second "meaningful vote" on the deal in the House of Commons this evening.

Shadow Brexit secretary Keir Starmer said Cox had confirmed that "no significant changes" had been secured in two months of negotiations and the Government's strategy was "in tatters".

READ MORE: Brexit Vote Live: MPs to vote on Theresa May's deal

The advice was issued the morning after May's dash to Strasbourg to finalise a deal with European Commission president Jean-Claude Juncker which she said would deliver "legally-binding" reassurances for MPs to ensure the Irish backstop cannot be permanent.

READ MORE: Chaos and confusion as May returns from Strasbourg with latest fudge

In it, Cox said that documents agreed in French border city "reduce the risk that the United Kingdom could be indefinitely and involuntarily detained" in the backstop by EU bad faith or a failure by Brussels to use its "best endeavours" to negotiate a permanent deal on the future relationship.

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Attorney General Geoffrey Cox

But he warned that the question of whether a satisfactory agreement on a future UK/EU relationship can be reached remains "a political judgment".

And he said that "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.

Starmer said in a tweet: "Attorney General confirms that there have been no significant changes to the Withdrawal Agreement despite the legal documents that were agreed last night. The Government's strategy is now in tatters."

And senior Conservative Leaver John Whittingdale told the Commons Brexit Committee that the Attorney General's advice was "pretty terminal" for May's plan.

Brexit-backing Tory backbencher Andrea Jenkyns tweeted: "Nothing has really changed, and it is still a bad deal so unable to vote for this."

Cox's advice was released as 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.

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The panel include the DUP's Nigel Dodds

The panel includes Nigel Dodds, the Westminster leader of the Democratic Unionist Party, whose judgment will be crucial not only in determining the position of his own party's 10 MPs but also many of the 118 Conservatives who rebelled against the Withdrawal Agreement in the first "meaningful vote".

The Prime Minister needs to win over scores of Tory MPs if she is to have any hope of reversing the 230-vote defeat she suffered when the Commons considered her Brexit deal in January.

She said she "passionately believed" her 11th-hour agreement had addressed concerns raised by MPs over the backstop, which keeps the UK in a customs union with the EU after Brexit to avoid a hard Irish border.

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 Cox to alter his advice on the indefinite nature of the backstop.

The changes secured by May do not alter the "fundamental legal effect" of the backstop and would not allow the UK to terminate it without EU agreement, found Lord Anderson QC, Jason Coppel QC and Sean Aughey.

Cox was due to make a Commons statement at around 12.30pm setting out his conclusions.

At a late-night joint press conference with European Commission president Jean-Claude Juncker, May said that three new documents provided the legal assurances critics of her stance had called for.

The PM said: "What we have secured is very clearly that the backstop cannot be indefinite. Cannot become permanent. It is only temporary. If it is the case that we were ever to get into the backstop."

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May added: "Now is the time to come together to back this improved Brexit deal and to deliver on the instruction of the British people."

On a day of high drama in Westminster, the Prime Minister chaired Cabinet from 9.30am before heading to the Commons to address Tory MPs behind closed doors. She was due to open the debate on her revised Brexit package with a statement on the floor of the Commons.

MPs are expected to vote at 7pm, with Environment Secretary Michael Gove saying it is "make your mind up time".

European Commission president Juncker insisted there would be no further negotiations on the issue.

"In politics, sometimes you get a second chance. It is what we do with the second chance that counts. Because there will be no third chance," he warned.

Ireland's Taoiseach Leo Varadkar said: "The further text agreed yesterday provided additional clarity, reassurance and guarantees sought by some to eliminate doubts or fears of some, however unreal, that the goal was to trap the UK indefinitely in the backstop.

"It is not - these doubts and fears can be put to bed."

Dutch Prime Minister Mark Rutte said he hoped MPs would back the deal as "there is no alternative".

The package includes a new "joint instrument" relating to the Withdrawal Agreement, setting out the legally-binding nature of both sides' promises to seek a permanent future relationship with "alternative arrangements" to remove the need for a backstop.

The Government says this document "reduces the risk" that the UK could be deliberately held in the arrangement indefinitely.

The second new document is a "unilateral declaration by the UK" which sets out "sovereign action" by which Britain could seek to have the backstop removed if the EU acted in bad faith.

The final document is a supplement to the Political Declaration "setting out commitments by the UK and the EU to expedite the negotiation and bringing into force of their future relationship".

If the package passes the Commons, leaders of the 27 remaining EU states will be asked to endorse the new documents at a scheduled European Council summit in Brussels on March 21-22, before the final step of ratification by the European Parliament.

Labour leader Jeremy Corbyn has called on MPs to reject the deal and accused May of a plan to "recklessly run down the clock" before March 29.