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MPs have one "last opportunity" to pass Theresa May's deal - or risk throwing itself on the mercy of the EU's other 27 member states, the Attorney General has warned.
Geoffrey Cox opened a debate on Friday morning by highlighting the EU had agreed an extension for Brexit until May 22 - but only if the Prime Minister's Withdrawal Agreement is passed by the House of Commons.
If it is not approved, the extension will expire on April 11 - and any further extension will need the approval of the leaders of all 27 EU countries.
"By this evening, if the 11pm deadline expires and the agreement has not been approved, that legal right [to extension] will expire with it," he said.
"This is therefore the last opportunity to take advantage of our legal right."
He went on to say that the motion acknowledges the Political Declaration - which previously has been tied in to the Withdrawal Agreement - is still "open to change" and that the EU has accepted it is open to change.
The Withdrawal Agreement argues the terms of actually leaving the EU, including terms on future trade and the Irish backstop - while the Political Declaration sets out plans for a future trade and security relationship with the EU.
It's the final throw of the dice for Theresa May to get her twice-rejected Brexit deal past MPs.
Speaker John Bercow on Thursday cleared the Government's Brexit motion on the EU Withdrawal Agreement for debate, ruling that it complies with parliamentary conventions which bar ministers from asking MPs to vote repeatedly on the same proposals.
He said the motion "complies with the test" because it is "new and substantially different".
The debate began at 9.30am, and MPs are expected to vote at around 2.30pm.
If Friday's motion is passed, the UK would qualify for an extension in Brexit talks to May 22 under the terms set down by the European Council last week.
But it would not fulfill the requirements of last year's EU Withdrawal Act, which stipulates that both elements (both the future relationship and "divorce deal" aspects) must be approved by MPs to pass the "meaningful vote" allowing the deal to be ratified.
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Yet soon after the vote was announced, as Labour said they would not be backing the Prime Minister's "blindfold Brexit" deal, while the DUP maintain they cannot vote for a deal which includes the controversial backstop.
If MPs do not approve the motion, the prospect of leaving the EU on April 12 without a deal becomes ever more real.
During the debate, former Tory leader Iain Duncan Smith said he would back the motion to get Brexit going, warning a longer extension on April 12 is "death in terms of our voters" and would mean the UK "will never leave" the EU.
"We need to have a moment where we recognise what we need to take hold of is that one element that gets us out, leaves us out, and then shuts down the debate about future referendums, and allows us to take the confidence that under a new leadership we can go forward to change the nature of what is in this process," he said.
"I, therefore, say to my colleagues that for me this is not an easy decision because there is a lot about this Bill that I do not like - and I stand on that position.
"But I do say if we don't send this forward to look at the legislation we will rue that day because we will end up having to accept what I think is a damaging and destructive extension - meaning we will never leave the European Union."
On the other side, former Wales Secretary John Redwood said he would be voting against the government - calling it a "dreadful agreement" which would trap the UK with no exit clause.
Mr Redwood likened the proposed arrangement to handing over money to a shopkeeper, and only then beginning a discussion about what one wanted to buy.
"For me, it turns out to be an easy decision. I am sorry for a lot of my honourable and right honourable friends for whom it is not so easy," he said.
"On this one issue, I have voted against the government before and I will vote against it again this afternoon because this is a dreadful agreement.
"It is a fully-binding treaty with no exit clause. Something we can't get out of, and there would be requirement after requirement after requirement.
"We would have sub-contracted our legislation to someone we can't control and we would have to obey. We would have offered to pay them a lot of money for no obvious good reason."
The debate began to rage even before MPs entered the House of Commons on Friday morning.
Former Brexit secretary Dominic Raab said he "does not feel guilty at all" that on what was supposed to be "Brexit Day" he will not be backing the Withdrawal Agreement - though added that he does not "advocate any delay" on the UK leaving the EU.
However, he changed his stance once inside the Commons, revealing he would in fact be voting in favour of the deal, with the possible delay to Brexit forming the basis of his new position.
"I cannot countenance an even longer extension and I cannot countenance holding European elections in May," he said.
"I will vote for the motion."
Also speaking before the debate began, Andrea Leadsom told MPs that the vote they face "really is the last chance and I urge all colleagues to take it" and "approve the Withdrawal Agreement which is what the EU needs us to approve to get the extension to get the legislation done", allowing the UK to leave the EU on May 22.
Boris Johnson was also among those to announce he would support Mrs May's deal - calling it a "painful" decision but one which was "better than the alternatives", including losing Brexit altogether.
Jeremy Corbyn confirmed his party would "vote against the motion", arguing the Government could not treat the Withdrawal Agreement and the Political Declaration as two separate documents.
"What the Prime Minister is trying to do is something she denied she would do on 14th January and that is separate the Withdrawal Agreement from the Political Declaration, from the future arrangements.
"Well you can't separate them, because otherwise you move into a blindfold Brexit on the basis of the Withdrawal Agreement.
"There's no way out of it once you've signed it and gone into it, and we're not prepared to support the Prime Minister on this."
The Democratic Unionist Party, too, was standing firm, confirming what it said on Wednesday - that it would not be backing a fresh vote since "the necessary changes we seek to the backstop have not been secured".
Deputy leader Nigel Dodds told ITV News Political Correspondent Daniel Hewitt: “This doesn’t change anything in terms of our position - it is a procedural way around, our concerns remain.”