Henry Newman is Director of Open Europe. Guglielmo Verdirame is Professor of International Law at King’s College London and practises at 20 Essex Street Chambers.

Last week, the EU managed to undermine its own position by arguing first that there would be a hard border in the event of No Deal and then that there would not be one – thus calling into question why the backstop is required in the first place. Recent comments by European figures, including Jacek Czaputowicz, Poland’s Foreign Minister, suggested a growing realisation that it would be ironic if a deal couldn’t be reached between the UK and EU, risking a harder border with Ireland, precisely because of a policy intended to avoid the need for such a hard border. Yet rather than sending the Prime Minister back to Brussels with a clear mandate to seek changes to the backstop, MPs risk giving a confused message in this evening’s votes.

Yesterday, ERG supporters denounced an amendment put forward by the 1922 Committee Chairman, Graham Brady, which would require the backstop to be replaced. There are now reports that agreement has been reached between backbenchers on possible alternative arrangements (‘the Malthouse Plan’), but it’s unclear why the EU would support these. Replacing the entire backstop is something the EU is unlikely to ever accept.

What can realistically be achieved? Although the backstop has various problems, the biggest is the weakness of its exit mechanism.  The Government must improve this. Policy Exchange argued in a report in December and a paper today that the UK must reserve its rights to leave the backstop, under the Vienna Convention on the Laws of Treaties, if the EU did not meet its commitment to use “best endeavours” to find alternative solutions.

The UK also needs clarity over what alternative solutions could replace the backstop. This is because of the European Commission’s recent insistence that a “subsequent agreement … would ensure the absence of a hard border on the island of Ireland on a permanent footing [our italics]”, and because the EU has so far indicated that there are no alternatives to the backstop, besides full membership of both the Single Market and a Customs Union, or a reversion to a Northern Ireland-only backstop with an Irish Sea customs border. It would not be a good faith interpretation of the EU’s commitment to find a subsequent agreemen were the only subsequent agreement to be substantively identical to the backstop. Nor would it be reasonable to refuse the UK any future relationship which had a unilateral right of withdrawal. It cannot be the case that the only way to replace the backstop is with a treaty with its own backstop.

Some MPs are demanding a time limit on the backstop (something Theresa May was denied at the December European Council). However, a short time limit could create a cliff edge, working to the UK’s disadvantage. As Open Europe has suggested, it could “add unnecessary time pressure to the negotiations on the future relationship.” So a clearer mechanism allowing the UK to leave the backstop, if the EU fails to negotiate in good faith on a future relationship, is preferable, not least because – as Paul Bew and others have argued – the backstop itself risks undermining the Belfast/Good Friday Agreement.

Writing in yesterday’s Daily Telegraph, Boris Johnson demanded what he called a “Freedom Clause”, saying it should be “either a sunset clause or a mechanism for the UK to escape without reference to the EU” and that “this means reopening the text of the Treaty itself”. But re-opening the text isn’t the only answer, nor necessarily the best.

The Withdrawal Agreement already has plenty of what could be called constructive ambiguity. Article 1 (4) states that the Backstop is “intended to apply only temporarily” and that the “objective … is not to establish a permanent relationship”). However, it also continues to say, “this Protocol shall apply unless and until they are superseded, in whole or in part, by a subsequent agreement”. Although it might be helpful to tidy all this up with changes to the text itself, that might be a tall order, when the withdrawal agreement, warts and all, has been signed off by all EU leaders.

A subsequent and more specific agreement with the EU clarifying things could be preferable to amending the existing treaty. In the event of a conflict between the two texts, the later and more specific one would prevail. And although some have dismissed the Exchange of Letters earlier this month between the UK and EU as legally “worthless”, they are incorrect. In international law, exchanges of letters or notes can be considered a treaty – see for example the 1940 and 1941 Land-Lease Agreements with the US. Even if the Exchange of Letters were to be viewed as just short of a treaty (i.e. as a Memorandum of Understanding or a Joint Interpretative Declaration), it would be far from legally worthless. It would be very difficult for the UK, the EU, or any tribunal, to proceed on the basis of an interpretation that is contrary to what was set out in that Exchange.

At this stage, an unfettered unilateral right of withdrawal may not be realistic. To address the exit from the backstop and what could replace it, the UK may need a further (and more robust) exchange of letters with the EU. International law does not have the same formal rigidity as domestic law, and MPs should be careful not to close the door to solutions that are politically achievable and legally viable. It is, in practice, possible to develop the Withdrawal Agreement and the backstop with a subsequent treaty – in the form of an exchange of letters or (as suggested by others) a protocol or “codicil”. But the UK is more likely to achieve a compromise, than a clear-cut exit clause. That would make the end-point one of “sub-optimal mutual reassurance”. Both sides would know that in a scenario where no progress is made on a replacement agreement, each would have an arguable case against the other.

But to stand a chance of getting further clarifications, the Prime Minister will need to persuade Brussels and member states that she will actually be able to convince her critics to back her amended agreement. That’s difficult when MPs, such as Marcus Fysh declare that even securing a time limit to the backstop “isn’t enough”. As long as the EU believes any concession will be in vain, they are unlikely to offer anything further. MPs have a key opportunity to send a message to the EU that there will have to be changes to the backstop – they should support the Brady amendment.