What’s the embarrassing thing the government wanted to hide from us all in its Brexit advice? I have an idea

Now Theresa May has lost the vote and the government is being held in contempt, we have to wonder: was the withdrawal agreement made in bad faith?

Well, what have they been hiding, then?

You have to wonder. The May government, routinely described now as the worst of modern times, tried – and predictably failed – to defy the House of Commons and risked inflicting another historic defeat upon itself.

And all for what? The attorney general, Geoffrey Cox, came to the House to give a flamboyant and apparently open assessment of the full meaning of the “backstop”. Legally – which is the point of legal advice from a government law officer – we are indeed in the EU customs union indefinitely, shackled to the radiator until such time as Brussels wishes to release the UK, to use emotive language. Politically, it may be that that won’t last “forever”, but that is politics, not international law.

Yet if Cox has already owned up to the full grim horror of the backstop, why bother resorting to such extremes to keep the full legal advice secret? Had the Commons not roused itself it would remain under lock and key for decades.

It makes one wonder.

Could it be, for example, that Theresa May and the cabinet asked their solicitor, Cox, whether, under international law, the UK could actually just walk away from an EU-UK treaty? Would the EU be entitled to take legal action? Or retaliate?

There was some talk about this much earlier in the Brexit process. Could the British, in other words, actually unilaterally say that they want to break their treaty? Can it be done?

There is, indeed, an international treaty about international treaties. The Vienna Convention on the Law of Treaties was ratified in 1980 and no one has much heard of it. It did, though, attract the attention of a House of Lords select committee during the initial debates about triggering Article 50. The basic point about the Vienna Convention is that, yes, the UK could walk away from a treaty with the EU (eg the EU-UK withdrawal agreement or the forthcoming potential free trade agreement) if it wishes. Article 70 of the Vienna Convention, entitled “Consequences of the termination of a treaty”, provides: “1. Unless the treaty otherwise provides or the parties otherwise agree, the termination of a treaty under its provisions or in accordance with the present Convention:

a) releases the parties from any obligation further to perform the treaty;

b) does not affect any right, obligation or legal situation of the parties created through the execution of the treaty prior to its termination.”

So, was Cox asked whether the UK could, in effect, just tear up its new EU treaties, and do so quite legally under this obscure international convention? If so, what did Cox advise?

Did he mirror the House of Lords committee’s view on the European Union’s relationship with the Vienna Convention?

“Importantly, the CJEU [European Court] has in the past relied on the Vienna Convention to interpret EU law. In a judgment in 2010 it stated that, while the Vienna Convention did not legally bind the EU, or all of its member states, provisions of the Vienna Convention that reflected customary international law were binding on the EU.

“The court has held that, even though the Vienna Convention does not bind either the community or all its member states, a series of provisions in that convention reflect the rules of customary international law which, as such, are binding upon the community institutions and form part of the community legal order.”

This is the kind of thing you’d expect the law officers to offer advice on. If they haven’t, you’d also wonder why not.

So, either way, it is potentially explosive, as it would mean the UK was open to a charge of operating in bad faith: effectively thinking about tearing up its EU treaties before they were even signed. That would not go down well in Brussels, or indeed in many pro-European quarters in the UK. Even if Cox and ministers claimed it was merely precautionary and they were obliged to explore very option, these excuses would be seen as incredible – precisely because ministers have gone to such extreme lengths to evade publishing the legal advice.

There have been many tussles between crown and parliament over the centuries. We even had a civil war to settle the issue of who’s in charge. Rarely though can the House of Commons exercise its rights of sovereignty in such a way, and with such rich ironies attached to it.

The May government is used to knocks, defeats, U-turns and embarrassments. But this allegation of breach of faith – if that is indeed what it is trying to cover up – will be in an international league of its own.