It was, Michael Gove and Boris Johnson were assured by Downing Street, a "meaningless" assurance that was needed to get a vital agreement with Brussels over the line.

But the relevant clause in the "joint report" deal agreed last year has proved to be anything but meaningless for the European Union, 10 months after it was originally used to help unlock the second phase of Brexit negotiations.

The controversy has centred on paragraph 49 of the document, which stated that "in the absence of agreed solutions”, to avoid a hard border on the island of Ireland, where the EU feared a 300-mile back door for goods into the bloc, “the United Kingdom will maintain full alignment with those rules of the Internal Market and the Customs Union which, now or in the future, support North-South cooperation, the all-island economy and the protection of the 1998 Agreement."

An alignment of rules was intended to obviate the need for checks on the Irish border, as goods flowing from the north to the south would already comply with EU requirements.

An earlier version of the clause, stating that Northern Ireland alone would remain in full alignment with EU rules, had been replaced at the insistence of the Democratic Unionist Party, which is propping up her government, after they learned about it at the eleventh hour. Arlene Foster, the DUP leader, made clear she would not accept Northern Ireland being tied more closely to the EU than the rest of the UK.