Jo Johnson supports his brother’s One Nation domestic agenda. He strongly approves of the new Prime Minister’s plan to recruit more police officers, teachers, doctors and nurses, while also promoting the dynamic economy needed to pay for them.

The new administration’s friendlier attitude to foreign students, and emphatic backing for science, likewise meet with his warm approval.

In 2016, when Boris Johnson announced at what was expected to be the launch of his leadership campaign that he would not after all be standing, there was Jo Johnson among the relatively small group of MPs who had turned out to support him.

And at this summer’s launch, there was Jo among the by now far larger number of MPs who had decided to back Boris, and who see him as a brilliant leader who will be a brilliant PM.

Why then is Jo standing down as a Conservative MP and minister? The answer for him as for a number of others is Europe. They are appalled by the risk of a no deal Brexit.

To them, the Conservatives ought not to be a party of ideological risk-takers.

Last November, Jo Johnson resigned in protest at Theresa May’s approach to Brexit. In his resignation statement, he said:

“Brexit has divided the country. It has divided political parties. And it has divided families too. Although I voted Remain, I have desperately wanted the Government, in which I have been proud to serve, to make a success of Brexit: to reunite our country, our party and, yes, my family too. At times, I believed this was possible. That’s why I voted to start the Article 50 process and for two years have backed the Prime Minister in her efforts to secure the best deal for the country. But it has become increasingly clear to me that the Withdrawal Agreement, which is being finalised in Brussels and Whitehall even as I write, will be a terrible mistake.

“Indeed, the choice being presented to the British people is no choice at all. The first option is the one the Government is proposing: an agreement that will leave our country economically weakened, with no say in the EU rules it must follow and years of uncertainty for business. The second option is a ‘no deal’ Brexit that I know as a Transport Minister will inflict untold damage on our nation. To present the nation with a choice between two deeply unattractive outcomes, vassalage and chaos, is a failure of British statecraft on a scale unseen since the Suez crisis. My constituents in Orpington deserve better than this from their Government.”

He went on to remark that he and his brother were “united in fraternal dismay”. But he also warned Boris that a “no deal” outcome simply would not do:

“A ‘no deal’ outcome of this sort may well be better than the never ending purgatory the Prime Minister is offering the country. But my message to my brother and to all Leave campaigners is that inflicting such serious economic and political harm on the country will leave an indelible impression of incompetence in the minds of the public. It cannot be what you wanted nor did the 2016 referendum provide any mandate for it.”

There is a striking strength of feeling in these words. Jo Johnson has spent most of his political career being singularly uncommunicative with the wider public, as I noted in a profile of him for ConservativeHome in 2013.

But that does not mean he lacks convictions. He has resigned because his profound loyalty to his brother cannot be reconciled with his profound opposition to a no deal Brexit.

The press interprets this as fratricide, betrayal, a Shakespearean tragedy. It is actually the most logical and honourable response to an intolerable conflict of interest.