Theresa May on Monday night accepted from the EU a package of assurances, including enhanced legal commitments on the backstop, that she said would allow her to put the Brexit withdrawal agreement to MPs on Tuesday.
The British prime minister was in Strasbourg for an unscheduled meeting with European Commission president Jean Claude Juncker. She left to return to the House of Commons where the final details were being put to the government's Tuesday motion or "meaningful vote" on the withdrawal agreement. Monday night's deal has also yet to be cleared by the cabinet.
Mr Juncker, at a press conference following the talks, appealed to MPs to back the agreement on Tuesday, warning that it was the last chance. There could be “no further chances”. It is “this deal or Brexit might not happen at all”, he warned.
The agreement will see a legal strengthening of the assurances given in January by the EU through a “joint legal instrument” on the impermanence of the backstop that Mrs May insisted would have “the same status as the withdrawal agreement”.
The text will also strengthen the ability of the UK to ask independent arbitrators in the event of deadlock caused by EU bad faith in negotiations to allow for the partial or total dismantlement of the backstop.
Amendments to the political declaration on the future relationship will recommit both sides to immediate talks on a deal which will include “alternative arrangements” which can replace the backstop.
Whether the deal will be sufficient for sceptical backbenchers will be determined on Tuesday. But to strengthen her hand Mrs May announced that if the EU would not grant a unilateral right to withdraw from the backstop the UK would itself claim that right.
" We will make a unilateral declaration that if the backstop comes into use and discussions on our future relationship break down so that there is no prospect of subsequent agreement, it is the position of the United Kingdom that there would be nothing to prevent the UK instigating measures that would ultimately dis-apply the backstop," she said.
That legal right to break the agreement unilaterally is not accepted by the EU but on Monday night Mr Juncker would not be drawn on his attitude to the declaration.
Irish sources say that the interpretation is a matter for the UK and ministers will be urged to avoid comments that might undermine support in the Commons.
The deal signed does not change the withdrawal agreement but provides what the UK sees as important, legally copperfastened assurances on the temporary nature of the Irish backstop.
EU ambassadors met on Monday in Brussels and expressed confidence in the approach taken by chief negotiator Michel Barnier and his team.
Sources say there was clearly a willingness to give the UK an extension on article 50 should there be a deal to allow time for the passage of necessary legislation. But attitudes to an extension in the absence of a deal were “hardening”.
Over the weekend officials from both sides led by the EU's Sabine Weyand and the UK's Olly Robbins worked through Saturday to broker an agreement. Mr Robbins travelled back to brief a cabinet meeting in London on Sunday where Mrs May found resistance to the proposals as they stood. She spoke to Mr Juncker by phone on Sunday evening and Monday morning.
The main focus of officials' weekend discussion was understood to have been proposals on assurances made by EU chief negotiator Michel Barnier on Friday evening. The EU, he said, was ready to strengthen the legal standing of a January joint letter to the UK from Mr Juncker and EU Council president Donald Tusk in which they gave reassurances that the backstop would only ever be applied as the last resort, and only temporarily.
He said the EU would affirm the right of arbitrators, provided for in the agreement to act in the event of a deadlock, to suspend part of the backstop if the EU was found not to be negotiating its termination in good faith.
And Mr Barnier said the agreement would allow the UK to withdraw from an all-UK backstop as long as it maintained a Northern Ireland-based backstop in order to protect the EU single market in Ireland.
The immediate and speedy specific repudiation on Friday evening of the latter element of the deal by Brexit secretary Stephen Barclay was seen as a sign that there might be room for movement on the other two issues.