The London mayor’s decision to back Brexit has electrified the campaign and is set to influence millions of voters, according to a snap survey on the so-called “Boris effect”.
City insiders believe the charismatic politician has transformed the prospect of the Leave campaign overnight.
And after Mr Johnson declared his hand, 99 per cent in a Daily Express phone poll said they wanted out.
The findings come just a day after Mr Johnson said he would defy David Cameron by calling for an exit vote in the historic in-out referendum on June 23.
Making his case for an “out” vote, he said: “This is a once in a lifetime chance to vote for real change in Britain’s relations with Europe. This is the only opportunity we will ever have to show that we care about self rule.”
The first evidence of Tory heavyweight BoJo’s impact came in a poll by BMG Research which suggested up to nine per cent of voters are willing to change their minds on Europe on the strength of what he says.
Dr Michael Turner, of BMG, said: “The Mayor is directly eating into the ‘Remain’ campaign. It is not direct switching, which I would not expect at this stage, but a significant group that had planned to vote ‘remain’ suddenly becomes undecided.”
Analysis from US-based bank Citi also said Mr Johnson’s influence significantly boosted the chance of a UK exit.
It was yesterday put at 30-40 per cent – up from 20-30 per cent before the weekend.
The support of Mr Johnson and Tory Justice Secretary Michael Gove had been the key factors in the transformation, according to the research.
Citi’s chief UK economist Michael Saunders said: “The Brexit side has lacked heavyweight figures in UK politics. That changed with the backing of Gove and, in particular, Johnson.”
Leading poll expert John Curtice, professor of politics at Strathclyde University, said senior figures would influence the vote outcome.
The academic, who came close to predicting the final result of last year’s general election, said: “Personalities matter because they are vehicles for the communication of messages.
“I’m not sure at the end of the day many people are going to vote on the basis of whether they like David Cameron or Boris Johnson or Nigel Farage or Philip Hammond as individuals.
“But they are the people who above all will be trying to persuade us which way to vote. And the effectiveness with which they do so will matter.”
Anti-Brussels campaigners continued to celebrate the recruitment of Mr Johnson to their cause yesterday.
Former Tory chancellor Lord Lawson, chairman of the Vote Leave referendum campaign group, said: “I am delighted he has come out for leaving the EU. He is a superb campaigner and a great asset to the cause.”
Mr Johnson, whose intervention severely rattled Downing Street, stepped up his campaign with a furious attack on Brussels during Mayor’s Question Time at the London Assembly.
Mr Johnson said: “There will always be people who say we should stick with the status quo.
“But the trouble with the status quo is that it is formidably bureaucratic, it is producing more and more legislation over which neither our parliament nor any parliament in Europe has any control.
“There are people who don’t think Britain could stand on our own two feet. I think that is profoundly wrong.
“I think the people who make these arguments are the same as the people who warned that we shouldn’t leave the ERM, which turned out to be the salvation of the UK economy; and they are the same as the people who said that we had to join the euro, which turned out to be a catastrophic mistake and a very unfortunate enterprise.
“I read plenty of people who think actually that the British economy could prosper outside the European Union, and not just the British economy but London and the City of London too.”
Mr Johnson’s father yesterday claimed his son had taken a “career-ending decision” by snubbing the PM.
Stanley Johnson, a former MEP and chairman of the Environmentalists for Europe campaign group, said his son made “a well-thought-out move… it represents his deep conviction”.
He added: “To say this is a careerist sort of move would be a total travesty. I cannot think of any more career-ending move. If he wanted to get a nice job in the Cabinet this is not the way to do it.”