Theresa May has returned from meeting EU Commission president Jean-Claude Juncker with what she insists are the “legally binding” changes to her Brexit deal that were requested by Parliament.
However, after the altered deal was scrutinised by the Attorney General (who offers the Government legal advice) he said the "legal risk remains the same".
Geoffrey Cox conceded that the Prime Minister's last-minute Brexit agreements "reduce the risk" that the UK could be trapped indefinitely in the backstop, but do not remove it altogether.
Mr Cox's legal advice deals a significant blow to Mrs May's hopes of overturning MPs' 230-vote rejection of her Withdrawal Agreement in the second "meaningful vote" on the deal in the House of Commons on Tuesday evening.
So, as the Prime Minister prepares to present the new deal to MPs ahead of a crunch vote, here are some key questions about the developments answered:
What has the Prime Minister achieved?
Three new documents were agreed with Mr Juncker on Monday night.
The first is a “joint instrument” that relates to the Withdrawal Agreement.
The PM said it reduces the risk that the UK could be “deliberately held in the Northern Ireland backstop indefinitely”.
It is on this which Mr Cox said the "legal risk remains unchanged".
The second is a “unilateral declaration” by the UK which sets out “the sovereign action the UK would take to provide assurance that the backstop would only be applied temporarily”.
The third 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”.
What does all that mean?
The joint instrument gives Britain the ability to suspend the backstop if the EU is deemed to have deliberately applied it indefinitely, although the UK would have to win the backing of an independent arbitration body to do so.
The unilateral declaration is a statement on the record that the UK can “instigate measures” to remove itself from the backstop if talks break down.
This could carry legal weight if it is lodged with the United Nations without formal objection by the EU.
The supplement to the declaration states the intention that both sides will set about finding an alternative to the backstop as soon as possible.
Will it satisfy those who opposed the deal first time round?
The Democratic Unionist Party (DUP) rejected the deal before over fears the backstop, if activated, would create a customs border between Northern Ireland and mainland Britain.
The European Research Group (ERG) of Brexiteer Tory MPs also rejected it because they feared the backstop would keep the UK too closely bound to the EU and unable to strike trade deals with non-EU countries.
They demanded the backstop was either removed completely from the Withdrawal Agreement or subject to a time limit.
Neither of those demands have been met, although the UK does now have the ability to suspend the backstop with the arbitration body’s agreement.
What have they had to say about it?
The DUP said they will be taking “appropriate advice” and “scrutinising the text line by line” before forming their own judgement.
The ERG’s lawyers will be doing the same.
Are they likely to reject it again?
For the DUP it is important that any Brexit deal does not effectively separate Northern Ireland from the UK, while Brexiteers want to leave the EU and take advantage of the ability to trade freely with non-EU countries.
For the DUP the backstop could present an existential threat, while for the Brexiteers it would hobble what they see as an opportunity for a more prosperous life outside the bloc.
The DUP may stick to their red line as a matter of principle if they deem the PMs changes to be insufficient.
Brexiteers could also reject the changed deal on grounds that the alterations do not go far enough.
However doing so could imperil the thing Eurosceptics want overall – to leave the EU – as rejection could lead to a delay to Brexit and raise the chances of second referendum.
How about Labour and The Independent Group (TIG) of MPs?
Labour leader Jeremy Corbyn said the Prime Minister’s negotiations had failed as the new agreement does not contain “anything approaching the changes Theresa May promised Parliament, and whipped her MPs to vote for”.
He said MPs “must reject this deal”.
TIG MP Chris Leslie said it was tabling an amendment to the motion that will force the PM to seek an extension to Article 50 for a so-called People’s Vote.
Will the EU consider any more changes or allow Brexit to be delayed?
On Monday Mr Juncker stated that there will be “no third chance” for more negotiations.
He did, however, leave open the option for an extension to Article 50 – although only until European Parliament elections are held in late May.