David Cameron has boasted that the deal he secured in Brussels with the 27 other European leaders is legally binding, and cannot be reversed.
He believes the promises in the deal have legal force, and that will be incorporated into EU law as part of treaty agreements in the future.
But in his first interview since announcing his decision to campaign to leave the European Union, the man who is responsible for the English legal system, Michael Gove, suggests that European judges could throw out the changes that Mr Cameron fought so hard to achieve.
And he urges voters to realise that the European Court still "stands above every nation state". He believes nothing in the deal will change that.
Given his role, and his closeness to the prime minister, Mr Gove's views have considerable significance in the early days of this fraught campaign.
With his customary sharp politeness, Mr Gove explained repeatedly how Mr Cameron's assertion that the deal can't be unpicked is essentially wrong.
He carefully stepped around making that direct accusation, saying instead: "There are two things which are true.
"The first thing is the prime minister is right: this is an agreement between 28 nations and all have agreed that they will abide by it.
"But above those nations sits the European Court of Justice."
But clearly, he, as now one of the senior members of the Vote Leave campaign, is disappointed that the prime minister's negotiations did not achieve his original aim of "full-on treaty change" and wants to highlight what he believes are the limitations of what's been achieved.
Mr Gove's comments will be catnip for Eurosceptics who've already been arguing that Mr Cameron's deal is hardly worth the paper it's written on.
For David Cameron, maintaining the sense that the deal he's achieved can and will improve the UK's relationship with the rest of the European Union in a meaningful way is a vital part of his campaign.
It's one thing Tories from different parts, different generations of the party, disagreeing. It's quite another when it is two who have shared political hopes, ambition, and even family holidays.
Michael Gove praised Mr Cameron's generosity in allowing ministers to disagree publicly. But politics has moved into a phase when friendships and loyalties will be tested like never before.
There'll be more of Mr Gove's interview here throughout the day on BBC News.
So it begins - not just rivals, but political friends of the closest kind, making strong but completely contradictory claims in the referendum campaign.