Earlier this week, the media reported that Geoffrey Cox had struck a new note in the negotiations over the Irish backstop: claiming that it could breach the European Convention on Human Rights.

According to the Guardian, the Attorney General argued that because Northern Ireland would be subject to a legislature in which it was unrepresented, British citizens there could say the arrangement breached Protocol 1, Article 3 of the Convention, which “protects the right of people to vote in order to choose their legislature”.

Nor was this new tone confined to press reports from the negotiations. On Thursday this week Gavin Robinson, the Democratic Unionist MP for Belfast East, asked the following question in the Commons:

“I understand that the Attorney General’s conversations with the Cabinet are privileged, but has he turned his mind to the concerns that, should the backstop be indefinite, it is likely to breach the commitments under the Belfast agreement, and indeed the commitments that are given to me as a Northern Ireland citizen under article 3?”

Cox’s reply was superficially evasive, but eye-opening:

“The hon. Gentleman knows that if I were to answer that question, I would be breaching the Law Officers’ convention. All I can say is that I turn my mind to a great many of the legal implications of the treaty, and those that he has mentioned have not escaped me.”

All of this paints a clear picture: that Cox, the expert lawyer charged by the Prime Minister with finding a solution to the backstop, has ended up taking a very different view of its compatibility with the Belfast Agreement to the official line laid down by Downing Street.

The root of this tension is perhaps best illustrated by the response offered by Lisa Chambers, a legally-trained TD and Ireland’s shadow Brexit spokeswoman, who accused the Attorney General of “trying to solve a political problem with a legal solution”, and challenged May’s decision to send “a lawyer to negotiate when it is a political, not a legal, problem”.

Ever since the Government first blundered into the backstop in 2017, there has been a great deal of praying-in-aid of the Belfast (‘Good Friday’) Agreement to justify this or that demand of the Irish Government – most notably a concerted effort to paint the maintenance of a completely invisible border as if it were mandated by the treaty, which it is not.

Neither May nor her lacklustre appointees to the Northern Irish Office have done much, if anything, to challenge this, despite mounting disquiet from both the DUP and Ulster Unionists and even a legal challenge from Lord Trimble, one of the original architects of the Agreement. Perhaps it suits the Prime Minister to short-circuit the debate over the backstop by dressing it up as an unavoidable legal obligation.

By taking a different tack, which is to say acting like a lawyer, Cox is finally offering the sort of sceptical, expert scrutiny of the backstop that London ought to have been offering from the start. This is to be welcomed, because a detailed push-back against Dublin’s ‘maximalist’ interpretation of the UK’s obligations is the Attorney General’s best bet of securing the sort of changes Brexiteer MPs need to back the Withdrawal Agreement.

The question remains, however, as to whether the Prime Minister will welcome and support this tough new line – or even whether, in a week’s time, her ministers are still directing the negotiations.