Theresa May's secret Brexit legal advice says exactly what we thought it said – just more bluntly

The prime minister resisted publication because she feels strongly about the principle of confidentiality. She made a grave political mistake

The attorney general’s legal advice says what the attorney general said it says, but in blunter language. The six-page document offers a fascinating contrast between private legal advice and the public summary of that advice – expanded to 43 pages.

The public summary puts the best possible gloss on Geoffrey Cox’s advice, but the short version is easier to understand and makes Theresa May’s dilemma starkly clear.

In the document published today, Cox spells out how the arrangements to keep the Irish border open are “intended to be permanent” despite the use of the word “temporary” in the protocol to the withdrawal agreement. He sets out how the provisions of the protocol “shall apply unless and until they are superseded” – and makes it clear that any agreement to replace the protocol has to have the same effect in keeping the Irish border open.

What is emphasised in the advice published today is what would happen if the negotiations to replace the protocol have “irretrievably broken down”. Even then, writes Cox, there would be no way to end the arrangements.

There is a paragraph in the attorney general’s formerly confidential advice that spells out that ending the so-called backstop is not merely a matter of relying on both parties to agree to do so in good faith. If the EU and the UK fail to agree that the protocol is no longer needed, he writes: “It is extremely difficult to see how a five member arbitral panel made up of lawyers who were independent of the parties would be prepared to make a judgment as whether the Protocol is no longer necessary, in the absence of the consent of the parties.”

This is a historically important document: the first time confidential legal advice from the attorney general to the government has been published while the issue was still live. It provides a fascinating case study in the difference of language between advice to ministers that the writer did not expect to see made public so soon, and summaries of that advice intended for public consumption.

However, the prime minister and the attorney general are justified in saying that the private advice does not contain anything of substance that is different from the public versions of that advice.

The government was foolish to resist demands for publication, because the headlines for the defeats in the House of Commons yesterday overshadowed the start of the debate about the Brexit deal. But I suspect that the prime minister resisted publication because she and other ministers, especially Cox himself, feel strongly about the principle of confidentiality, as do senior civil servants. They argue that it is impossible to run a good government if people are unable to have candid discussions in private.

As an abstract principle, that is probably true, but it is too easy to make the argument that this is a special case. And, indeed, it was an argument that persuaded MPs as they voted to find the government in contempt of parliament yesterday.