Boris Johnson says the Government's "doom-laden" predictions over the economic consequences of Brexit are in danger of "becoming self-fulfilling".
Mr Johnson dismissed the latest Treasury forecasts of post-leave recession, saying the "whole thing's a hoax".
"My own feeling is that there is some risk now that the prophecies are so doom-laden that they are at risk of becoming self-fulfilling," Mr Johnson said.
"I am worried that they are starting to talk Britain down quite significantly. I don't think it's necessary."
He said: "They think that this type of scaremongering is the best approach.
"I just remind you that the Chancellor himself pointed out that Treasury forecasts were politicised, they weren't necessarily reliable and that's why he set up the Office for Budget Responsibility.
"I think myself that the whole thing's a hoax."
He said: "I see no evidence whatever that there will be a recession, I really don't. It's totally made up. I think it's starting to be excessively negative and I think they're in danger of talking this country down."
Mr Johnson added that Prime Minister David Cameron had said only a few months ago that Britain would thrive outside the EU.
He said: "The Government's prognostications of gloom have got to be seen in the light of what they've said in the past, where the Treasury has repeatedly got things wrong, and in the light of, I think, the Prime Minister's much more credible assurances that, upon a departure from the European Union, we would readily do free trade deals around the world and that we have nothing to be afraid of."
Asked about supporting Turkey's EU accession 10 years ago, Mr Johnson said: "I think that 10 years ago the EU was a very different institution and, if you look at the way it's changed, particularly since the Lisbon Treaty, it has become more centralising and much more of a federal structure."
He said: "The Government is simultaneously claiming that they are the most fervent advocates of Turkish accession and that they propose to build a road, pave the way from Ankara to Brussels, while telling us that this is not going to happen till the year 3000.
"So I'm not quite certain what the Government's position is."
Mr Johnson also defended the "We send the EU £350 million a week" slogan on the side of the Vote Leave bus.
He said: "The number on the side of this bus is entirely reasonable, actually it's an underestimate. I think it's £365 million now. This is the total sum which we do not control every week that goes to Brussels.
"That figure I will defend. It's a perfectly reasonable figure to use. It's a fact. It's happening now. You can verify it."