Short of a resignation from one of the three ‘Great Offices of State’, it is difficult to think of a resignation more embarrassing for Boris Johnson than the one which has broken today.

Jo Johnson, the education minister, has announced that he is stepping down both as a minister and, come the election, as a Member of Parliament too.

Were that all, the speculation would be bad enough. But in a stinging rebuke to the Prime Minister he has couched his departure explicitly in terms which suggest he believes the current Government is not serving the national interest.

This will only add to the sense that his brother’s premiership, which got off to what looked like a fine start during recess, has started to fall apart once Parliament returned.

It will also deepen the impression that the Prime Minister is overseeing an evolution of what the Conservative Party represents, and perhaps harden the divisions between the rebels and the leadership which might, in the right circumstances, have been bridged.

There is no doubt that Johnson (B) tried very hard to minimise the risk of resignations. The pursuit of this strategy led him to conduct one of the most brutal reshuffles in modern political memory, sending to the backbenches not only determined opponents of his policy but others, including the likes of Penny Mordaunt and Liam Fox, who were on board.

Whether he knowingly took a gamble for the sake of family, or simply miscalculated, the Prime Minister once again finds the headlines stolen by another Johnson.

The stakes are growing ever-higher for him, his Party, and his Government. Internal pressure from is opponents is growing – just this morning the papers reported a substantial push, led by Cabinet ministers, for the reinstatement of the ‘Whipless 21’. The Prime Minister struck some sort of deal to call of the Lords filibuster last night but we don’t yet know if he has secured an election before October 31. There are calls for the heads of his senior team, not least Dominic Cummings.

But he surely knows that to give in to these demands means the end of his strategy, his premiership, and perhaps even Brexit. There would be no point in the Conservative Party fighting a close-run election on a platform for which a score or more of its MPs will not vote – and which some are on record as being prepared to install Jeremy Corbyn to prevent. If Johnson follows May’s example in sacrificing party discipline and his key advisers, he will likely meet her fate.

One way or another, a showdown looms.