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Cutting a birthday cake iced with “Back Boris”, the man likely to become prime minister next month noted with satisfaction: “It’s blue all the way through.”

The frontrunner in the Tory leadership contest then gave a brief pep talk to 20 or so keen young volunteers crowded into the basement of his campaign headquarters yesterday, his 55th birthday. Not the meandering, joke-strewn stump speech full of surprises that his fans adore, but a focused and serious message. “We are by no means there yet,” he said, warning that the next phase of the Tory leadership contest will be the toughest. “This movement is going to have to build.” So has the old Boris been retired? The one who made Londoners and the nation laugh with him (and sometimes at him) as eagerly as they voted for him?

Giving his first interview to a daily newspaper since the campaign began, Mr Johnson was clear that he is no longer playing for laughs. “Well, I do think it is time we had some excitement back into politics, I do, but there’s also quite a serious job of work to be done.”

The former London mayor was sitting in the study of 11 Lord North Street, the home of Tory peer Lord Howard of Rising and, coincidentally, the address where Michael Portillo supporters tried to organise a botched leadership contest 24 years ago. It is rare for a reporter to see the nerve centre of the Johnson For PM campaign. A colour map of parliamentary constituencies leans against the wall, while downstairs computers on borrowed desks sit incongruously under oil paintings on the walls.

“If you look at what the Brexit vote was all about, it was this,” Mr Johnson said, mapping out what he sees as the task ahead.

“There are lots of parts of the UK that just feel left behind, they feel they don’t share in the success of this incredible country. And yes, it was about democracy; yes, it was a bit about immigration, no question. But it was also about a sense that parts of Britain don’t get the attention, don’t get the investment, don’t get the opportunity that they could.”

Mr Johnson's constant refrain is that he wants to be “the prime minister who does for the whole country what I did as mayor of London”. What priorities does that signal?

“It’s the same formula: it is education, infrastructure and technology — those three things. I want to level up education funding across the UK. There are too many bits of it that don’t get anything like the per-pupil funding as London. And we need to be doing far more with transport infrastructure. I would like to be the prime minister who does for the Northern Powerhouse rail and connectivity with the West Midlands what I was able to do with the Tube upgrades and Crossrail.”

Outside a chant of “Boris!” wafted up from a flashmob of demonstrators that followed his car from the Commons and arrived wearing European Union hats and waving anti-Brexit banners. Mr Johnson’s ears pricked up but he just chuckled.

Do the continuing Brexit divisions worry him? “Yes, and the way we need to heal them and get this thing done is to get Brexit over the line, deliver it for the people and then concentrate on modern Conservative policies.”

He talks a lot about “reaching out” beyond the Tory Party’s traditional hunting grounds for votes, to appeal to more diverse communities, including minorities and people on the lowest incomes. “We put millions into the pockets of the poorest around this city with the living wage and I want to do the same across the UK,” he said.

Labour heartlands, he believes, are ready for a Tory message providing Brexit is settled. “I think we can recruit voters from everywhere. I think many Labour voters have no interest in the metropolitan obsessions of Jeremy Corbyn with Venezuela or, you know, neo-Marxist economics. The Corbynista anti-Semitism leaves people totally bewildered and cold.” He said the Conservative Party’s relationship with black and minority ethnic communities has “changed dramatically in the past 20 years ... but we have got to turbocharge that now and do much more”.

Mr Johnson wants his government to be seen as grateful for “the good” that he says immigration has done for Britain. “I have always been a believer in immigration and in allowing talented people to come to this country.

“This is a country of incredible opportunity. My great-grandfather who was Turkish came here because it was open and generous, and I want to make sure that the Conservative Party is the party of opportunity for everybody.”

A familiar protester’s voice bellows from the street below, “Bollocks to Boris”. Mr Johnson’s eyes narrowed. “You can pour water on that one,” he growled. Confusingly, the crowd then struck up Happy Birthday. “What is going on here?” he chuckled.

What is the first phone call he will make from No 10 to launch his Brexit plan? Mr Johnson gave no answer, instead launching into his familiar riff about wanting to “get Brexit done by October 31”. Exactly how he will manage it remained mysterious.

Will former Brexit secretary and ERG poster boy Dominic Raab, who endorsed him in yesterday’s Standard, be on his negotiating team? “There’s a lot of good people who will be on the team,” said Mr Johnson, dodging the question.

What about a promotion for Rory Stewart, whose brief Beatlemania was halted in last night’s vote? “I have been absolutely scrupulous in not making promises,” said the frontrunner.

Will he advance the northern economic powerhouse by signing off HS3, the fast rail link connecting the great cities with the West Midlands and London? He repeated his commitment to boost northern infrastructure as he did in London, a vague promise that will leave pessimists worrying the cheque will never arrive.

Asked if he would stand in the way of Heathrow’s third runway, the former mayor who once vowed to lie in front of a bulldozer to stop it replied: “I have grave concerns about the ability of the promoters of the third runway to meet their legal obligations on noise pollution and air quality. As you know, there are court cases under way.”

His answer suggests he will not try to reverse the permission, which was voted through Parliament a year ago, but hints that he will keep the operators on a tight leash. “They have some very serious thresholds still to cross,” he said, when asked if he would be monitoring them closely.

Asked to confirm that he has promised Kent MPs in private that he has killed off his beloved Thames estuary airport scheme, which was supposed to be an alternative to Heathrow expansion, Mr Johnson’s face clouded into a picture of disappointment. “Suffice to say, that is not on the agenda,” he said.

He stressed his keenness to give a tax cut to the poorest by lifting the threshold of national insurance, a promise rushed out to repair the damage caused by an ill-judged pledge to cut taxes for the well-off.

He also boasted with a Trump-esque fanfare: “I was the most pro-business mayor you could possibly imagine.” He went on: “You’ve got to stick up for wealth creation and the free market. But you’ve got to do that because that’s the way to pay for the great infrastructure and the most needy in society.”

So what should be carved on Boris Johnson’s tombstone? Here lies a Brexiteer? Or should it be RIP a One Nation Conservative? “I think those two things are completely coherent,” he said, deadly serious. “I hope they will say, ‘There was somebody who helped to unite the country and unite society’. That’s what I want to do.”