David Cameron has bowed out with grace. Now it is Boris Johnson's moment

This is the biggest disruption of British politics since the Labour Party eclipsed the Liberals after the First World War. It is the end of David Cameron's career, which has ended as political careers so often do in failure. He gambled and lost.

He addressed the country this morning with directness and grace, saying that he would stay on as Prime Minister while a successor is elected, which would happen by the Conservative Party conference in October.

It was the right decision, respecting the democratic instruction of the British people. They have chosen to leave the EU and they are entitled to have as prime minister someone who believes in that course. It was at that moment this morning towards the end of his speech that it seemed to come home to him that this really was the end, and he struggled to hold back the tears. And it was at that moment that many people – not all of them Conservatives – must have wondered what they have lost.

He knew he had to go. The letter from Conservative MPs urging him to stay on, even in the event of a Leave vote, was for show. The party was happy to pretend to be united behind him – as long as they knew he wouldn't be there long.

There are immediate practical questions. He said the decision to activate the two-year period for negotiating the terms of the UK's departure from the EU was one that he would leave to his successor. Again, that was the right course.

That successor is almost certain to be Boris Johnson. There has been some talk in Westminster this week of Theresa May, the Home Secretary, as a "unity" candidate, but it must be remembered that the final choice of Conservative leader is made by party members as a whole. About 60 per cent of them will have voted to Leave, and they think she let them down by staying on the Remain side.

No, the leader has to be a Leaver, and there is no other Leaver who could credibly be prime minister and who is likely to be chosen by the party members. If it is Johnson versus May, Johnson would win. He is not only the most popular Leaver, he is the most popular politician in Britain. The only other serious candidate is Michael Gove and he genuinely doesn't want it – he wants to be Johnson's Chancellor.

A Johnson government would not be as different ideologically from Cameron's as many people assume. Johnson is just as much a One Nation Conservative as Cameron. They differ only on Europe. Indeed, in the referendum campaign Johnson presented himself as the champion of the working person, a denouncer of bankers' greed and an advocate of egalitarianism. He would not bring Nigel Farage – or Douglas Carswell – into government.

We are in a new world, but much about it will be the same as the old world.