Mr Johnson addressed a Vote Leave rally at York Racecourse, having earlier taken the Vote Leave bus to the Ginetta sports car factory in Leeds.
During the visit, he branded the latest financial warnings over Britain leaving the EU “a hoax”.
The former mayor of London said figures released by the Government in a bid to convince voters to remain in the EU at the upcoming referendum risked becoming a self-fulfilling prophecy.
Boris in Yorkshire
He said the warnings issued by Chancellor George Osborne with the Prime Minister’s backing were so “doom-laden” that they were at risk of becoming reality.
They include Britain’s GDP shrinking by 3.6 per cent and the loss of 820,000 jobs and unemployment leap of 2.4 per cent in the worst economic situation.
He said: “I think myself that the whole thing’s a hoax.”
He believes there will be a period of economic calm, and “then I think people get used to the new situation”.
Britain entering a year-long recession, dubbed a “DIY recession” by the Chancellor, should not happen, Mr Johnson claimed, nor should it have a bearing on the strength of the pound.
Speaking to The Yorkshire Post, Mr Johnson said that the pound need not take another dip like it did in February when it hit a seven-year low after he announced he was joining the Leave camp.
He said: “The pound has been tracking down for 18 months. I don’t think it’s any lower now than it was.
“The pound’s strength depends on the UK economy. The pound will go up. The only thing that worries me is the Government continually talking down the economy and Britain.”
On the Treasury’s latest warning, he said: “I just think that these types of prophecies are not credible – you have to ask yourself why on earth would the Government call a referendum if they genuinely thought that was the case?
“It makes absolutely no sense at all – nobody could conceivably take such a risk with this country. That’s not, in my view, believable.
“I remind everybody of where the Treasury has got things wrong in the past and what they said in 1992 about leaving the ERM – they said it would be a disaster, interest rates would soar, it would be economic mayhem.
“That turned out absolutely not to be the case – the reverse was true; it was a period of sustained growth and recovery.
“I understand that is their tactic, they think that this type of scaremongering is their best approach.
“I just remind you that the Chancellor himself pointed out that Treasury reports were politicised, they weren’t necessarily reliable and that’s why he set up the Office of Budget Responsibility.
“I think that the authority I am prepared to listen to is Steve Hilton (former aide of David Cameron), who did an excellent piece.
“It got to the heart of what the problem is, the democratic problem. He really elucidated that very well.
“The second authority is the Prime Minister, only a few months ago who was saying of course we would do well outside the EU, and of course there was going to be a lot of scaremongering – I think he was on the money then.”
Asked whether he thought the Government’s repeated warnings risk voters losing confidence in them and the case for Remain, he said: “My own feeling is there is some risk now that the prophecies are so doom-laden that there is a risk of them becoming self-fulfilling.
“I am worried that they are starting to talk Britain down quite significantly.
“I don’t think it’s necessary.”