David Cameron has accused Brexit campaigners of promoting a vision of life outside the EU that was "too good to be true", and warned that a vote for withdrawal would instead endanger trade and jobs.
The Prime Minister's warning came as Boris Johnson appealed to voters to ignore the "pessimists" and "gloomadon-poppers" and opt to quit the EU in the referendum on June 23.
Making his first speech on a Vote Leave platform since declaring for Brexit, the London Mayor said withdrawal from the EU could usher in a new era of prosperity for the UK.
"I think it is time to ignore the pessimists and the merchants of gloom and to do a new deal that would be good for Britain and good for Europe too," he said in a speech to workers at a transport depot in Dartford, Kent.
"It is time to burst loose of all those regulations and get out into a world that is changing and growing and becoming more exciting the whole time.
"If we hold our nerve and we are not timid and we are not cowed by the gloomadon-poppers on the Remain campaign and we vote for freedom and for the restoration of democracy, then I believe that this country will continue to grow and prosper and thrive as never before."
His comments appeared to be a direct rebuff to Mr Cameron, who warned on Thursday of potential large-scale job losses if Britain left the EU and accused the Out campaign of treating it as a "price worth paying".
But the Prime Minister stuck to his guns in a speech just hours later to the Welsh Conservative conference in Llangollen, telling delegates that the Leave camp were behaving as if EU withdrawal was an "abstract question" rather than something with direct and concrete consequences for ordinary families.
"They are asking us to trust that leaving would somehow be worth the profound economic shock and the years of uncertainty that would follow," said Mr Cameron.
"They say we would have more control. How exactly? Leaving the EU but remaining in the single market doesn't give us more control, it just stops us from having any say over the rules of trade. Relying on World Trade Organisation rules doesn't give us more control, it just hurts industry, it hits jobs and hikes up prices. Trying for a free trade deal doesn't give us more control, it just means years of painful negotiations and a poorer deal than we have today.
"In the end those who want us to leave are telling you that you can have all the benefits of EU membership but none of the trade-offs. But as everyone knows, if it sounds too good to be true, that's normally because it is.
"And let us remember, this isn't some abstract question. These are actually people's jobs, people's livelihoods, people's life chances, people's families we are talking about. I say `Don't put them at risk, don't take this leap in the dark'."
Mr Johnson appeared to endorse Canada's arrangements with the EU as a potential model for Britain in the future.
Asked whether the UK would have to accept free movement of labour as part of a post-exit deal, he said: "I don't think that is necessary. I think we can strike a deal as the Canadians have done based on trade and getting rid of tariffs."
Mr Cameron's warning of the risks of withdrawal came hot on the heels of a call from former prime minister Tony Blair for the Remain campaign to start making the positive case for EU membership.
"I would like to see the pro-European side get out there with a bit of passion and vigour and determination and stand up for what we believe," Mr Blair told the BBC Radio 4 Today programme.
"And what we believe not just as a matter of economic realism, but as a matter of political idealism."
Mr Blair signalled that he would not play a central role in the campaign, acknowledging that his participation "carries with it negatives as well as positives".
Strains within the Cabinet were again highlighted as the pro-Brexit Work and Pensions Secretary Iain Duncan Smith poured scorn on the warnings of the In campaign.
"They are almost panicky really, I listen to these endless comments and speeches about the dire warnings, they are almost biblical, you are expecting a plague of frogs and the death of the first-born," Mr Duncan Smith told the Today programme.
Downing Street, however, insisted that Mr Cameron was right to underline the economic risks involved in leaving.
"He thinks that it is important that the Government puts out the facts and the case to the British people," the Prime Minister's official spokeswoman said.
"When the Government has been so focused on making sure that we deliver economic and financial security for people up and down the country, it is important that we highlight the risks of leaving."