A Eurosceptic Cabinet minister has accused David Cameron and pro-EU campaigners of displaying "a low opinion of the British people" by downplaying the UK's prospects outside the EU.
Work and Pensions Secretary Iain Duncan Smith launched a blistering attack on those campaigning for a vote to Remain in June's referendum as he called for an end to personal attacks amid increasingly bitter fighting within the Conservative Party.
The Prime Minister had led a fresh assault on pro-Brexit campaigners, using a newspaper article to accuse them of wanting to take "the gamble of the century" with the UK's future on the basis of only "extremely vague" proposals.
But Mr Duncan Smith - one of five Cabinet ministers who have broken ranks to join the Leave campaign - insisted a post-exit favourable trade deal with the rest of the EU is "very doable".
He dismissed the potential impact of the emergency brake curb on migrant workers' benefits secured by the PM as part of the renegotiation deal with the other 27 member states, and he vowed to fight a ban on anti-EU ministers seeing official papers relating to Brussels.
"I have never heard such a lot of pessimistic downsizing of Britain's aspect," he told BBC1's Andrew Marr Show after Mr Cameron began a tour of the UK urging voters not to take a "leap in the dark".
Mr Duncan Smith added: "Britain is a phenomenal country, the fifth largest in the world. It has stood alone and fought for freedom, it has been a global trader, it can yet again be a global trader.
"Why would we have such a low opinion of the British people that we go out and talk about leaping into the dark, we talk about profound shocks, we talk about them not being capable, we're too small.
"I have a different view. My view is that Britain is a great country, the people here are inventive, innovative and they will find a way with us to actually have a real deal that gives Britain access to the world and access to Europe.
He went on: "The 'in' campaign's whole strategy seems to be 'ooh, it's terrible, hang on to nurse for fear of something worse'. It's been about saying basically we are too small, we are too little, we are too inconsequential, we can't do what we want.
"I do not know why anybody would want to run a country like this. This country is the greatest on earth."
He said Mr Cameron deserved credit for allowing himself and colleagues the freedom to oppose the official Government policy of remaining in the EU without being sacked, but he called for an end to personal attacks, saying: "Don't play the person; play the man."
Tory backbencher Rehman Chishti, who is yet to decide how to vote, told the Sky News Murnaghan show that Mr Cameron's strident criticism of Brexit supporter and London Mayor Boris Johnson in the Commons was "undignified".
Mr Duncan Smith - who said he has "deja vu" as a veteran of the rebellion against then premier John Major over the Maastricht Treaty - complained that ministers were undermining party unity by "briefing off" about the fate of colleagues.
Cabinet Secretary Sir Jeremy Heywood faces a grilling by MPs on Tuesday over his edict, backed by Downing Street, that it was "not appropriate or permissible" for officials to supply access to EU-related papers to ministers opposing the official line.
The Work and Pensions Secretary told the Sunday Telegraph he "must have the right to continue to look" at material such as the implementation of the "emergency brake", stressing he was "constitutionally" in charge of the department.
Writing in the same newspaper, the PM said: "When the people campaigning for 'out' are asked to set out a vision outside the European Union, they become extremely vague. It's simply not good enough to assert everything will be all right when jobs and our country's future are at stake."
He challenged them to set out what the trading relationship would look like, how long the economy would face uncertainty while it was negotiated, how joint-security arrangements would be replaced and how Britain's role and influence in the world would be maintained.
"With so many gaps in the 'out' case, the decision is clearly one between the great unknown and a greater Britain," the PM said.
"A vote to leave is the gamble of the century. And it would be our children's futures on the table if we were to roll the dice.
At least 130 of the 330 Conservative MPs have publicly declared their intent to defy Mr Cameron and back Leave in the referendum campaign.