Garvan Walshe is a former National and International Security Policy Adviser to the Conservative Party.

The trouble with international agreements is that they are, well, international. By this I mean that sovereignty can’t be used as a trump card in the way that parliamentary sovereignty can in most domestic law.

This strongly suggests that the Government’s course of action in seeking, in the disarmingly frank words of Brandon Lewis, “to break international law in a specific and limited way” has not been properly thought through.

The domestic problems with this approach are well known. Any legislation to break international law will run into trouble in the Lords, which will feel entitled to block it, as it was not only absent from the election manifesto, but in fact directly contradicts its promise to implement the Brexit deal sealed in November 2019.

There is also the matter of the ministerial and civil service codes, which forbid the breaking of the law (the removal of the word “international” from the code makes no difference in practice), and are likely also forbid actions openly directed towards that aim.

It is therefore an open question of constitutional law whether legislation to this end, introduced improperly by ministers, and drafted by civil servants would be valid. It is rather clearer that ministers or officials participating in the production of such legislation risk falling within the ambit of the common law offence of misconduct in public office.

But my concern here is international. The foundation of international law has long been that states are sovereign. As well as meaning that they begin with full powers to arrange their internal affairs, it also means they have the power to make agreements with each other. An agreement means that the states accept obligations to each other, which is what makes a treaty different from a state making a unilateral declaration to itself. While a state retains the practical power to break an international agreement, it cannot change the meaning of the agreement on its own.

It is also a consequence of this sovereign power that states are able to revise treaties they make, by mutual agreement, and it is of course often the case that this revision is dictated by power politics, but even that is different from mere reneging on a treaty. Nevertheless, the power of revision is usually held collectively by the states that signed the agreement, not by individual signatories. Some treaties, like indeed the Treaty of European Union provide an exit mechanism (Article 50), but others, like the Withdrawal Agreement, do not.

The Government might have been better placed to argue that it was trying to use its residual sovereign power to seek to renegotiate the Withdrawal Agreement, which it had concluded under the duress of the two-year withdrawal period contained in Article 50.

While that would probably not have gone down well in Brussels, openly seeking to break these particular parts of the withdrawal agreement is rather more challenging, because Michel Barnier’s team built in three levels of safeguards against what it would consider to be “perfidious Albion.”

First, the relevant aspects of the Ireland/Northern Ireland protocol are governed by EU law, interpretable by British courts and, ultimately, European Court of Justice. Because of the way the UK incorporates treaties (including treaties that give effect to legal systems like the EU’s) into its domestic law, sufficiently explicit legislation could probably escape disapplication by UK courts.

But this in itself would be a direct violation of the agreement, which the European Court could be expected punish with a fine. Though the UK could refuse to pay the fine, on the grounds that it was acting according to it own law, this would just trigger the second level of dispute resolution, which is the Joint Committee established to be established under the agreement.

If the Joint Committee cannot resolve the dispute to both sides’ satisfaction, and in this case it is hard to see how it could, the case would be submitted to an arbitration panel at the Permanent Court of Arbitration. Lewis’s declaration that the UK intends to “break international law” is unlikely be helpful to the British case.

Now, the UK may as a sovereign state in practice refuse to abide by the arbitration panel, but in that case the agreement (Article 178, paragraph 1) provides for the panel to “impose a lump sum or penalty payment”.

If the UK refuses to pay that, the subsequent paragraph allows the EU to suspend either parts of the Withdrawal Agreement with the UK, or of other agreements it has. These include agreements on aviation freedoms, equivalence for financial services, “data adequacy” vital to the tech sector, and the right of truck drivers to travel to the EU. This would amount to the “no deal Brexit” that the Prime Minister’s withdrawal agreement, endorsed in the 2019 general election, was supposed to avoid.

Now that the Government does not need the votes of the DUP, it should think carefully about whether it would rather spend the rest of its term engaging in an optional legal fight with the EU, or, having got Brexit done at the end of the year, stick to running the country it was elected to govern.