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.
Mr Duncan Smith said the UK market is so important to other EU businesses - such as Germany's car industry - that other member states "have a real requirement to trade with us, a desperate one".
"The Prime Minister says there is a challenge to know what life outside looks like. My answer to this challenge is very simple: we do a deal with the European Union.
"That is a trade deal, about access to our markets, access to theirs. Part of our red lines will be about us being able to control the borders as we want. We want migration but we want it as controlled migration so we can cope with it.
"That deal is very doable."
The EU faces "utter chaos and confusion" as its border rules collapse under the weight of huge numbers of refugees fleeing the Syrian civil war and other conflicts, he said.
"You've got Hungary putting up barbed wire, you've got the Macedonians refusing to accept anybody from Greece. This is a shambles.
"If this is the future under the European Union, we'd be better off saying, 'this is how we are going to cope'."
Mr Duncan Smith said the restriction on pro-Brexit ministers' access to material "can't possibly apply in the sense of us not knowing what is going on in the department because we are responsible for the department".
"For example, I will have to work on these proposals which came back from the recent negotiations."
Sir Jeremy is due to appear before the Public Administration and Constitutional Affairs Committee on Tuesday to be pressed over critics' allegations that it breaches neutrality rules.
Mr Cameron insists that while he has given ministers the right to campaign on the opposing side in a personal capacity, the Government has a formal position and the civil service must work to deliver that policy.
Commons leader Chris Grayling - who along with Justice Secretary Michael Gove, Northern Ireland Secretary Theresa Villiers and Culture Secretary John Whittingdale make up the Cabinet "leave" group - warned Mr Cameron against a "purge".
Mr Grayling said there should be "no recriminations" from either side following the referendum, amid reports that Tory MPs may try to oust the PM even if the UK votes to remain in the EU.
The Commons Leader told the Murnaghan programme: "I have to say I don't believe at the moment the Prime Minister is sitting in Downing Street plotting sackings of ministers.
"I mean, it would not be at all sensible when the referendum is over for there to be a purge of people on either side of the argument.
"If we are going to come together, if we are going to work harmoniously as a party, and if above all we're going to beat the Labour Party and win in 2020, we've got to come together well and there cannot be recriminations at the end of this.
"The Prime Minister has done the right thing in giving us all the freedom to debate on one side of the argument or another and I really admire him for doing that.
"All of us involved in that debate must make sure afterwards there are no recriminations.
Mr Grayling added: "I'm not in this because I'm trying to keep my place in the Cabinet or I'm trying to put in danger my place in the Cabinet.
"I'm doing this because it's the right thing to do."
Shadow business secretary Angela Eagle played down warnings that Labour leader Jeremy Corbyn - who Mr Duncan Smith said had previously been "utterly opposed" to EU membership - was failing to campaign strongly enough for the "remain" camp.
The Oppositon is "united in our determination" to secure victory in June, she told Marr, with no more than handful of MPs backing Brexit, and Mr Corbyn recognises that "it's Labour votes that'll keep us in Europe".
Mr Cameron's internal battles are the price he pays for a "cynical manipulation" of the EU issue, she said.
A Downing Street spokeswoman said: "There is no ban on ministers who support Leave receiving Government papers. Day to day EU business will continue to be conducted in the usual way and all ministers will retain access to any papers relevant to their departments.
"It is only papers specifically relating to the EU referendum that they will not receive and that reflects the fact that the Government has taken a position and they are choosing to take a different one."
The Britain Stronger In Europe campaign said its opponents had suggested the UK should seek to emulate the arrangements of more than 20 other countries - including Vanuatu, Nicaragua, Peru and Macedonia - if it left the EU.
Foreign Secretary Philip Hammond said the dossier of compiled alternatives showed they were unable to " set out a clear vision of what Britain would look like outside the EU".
"The Leave campaign has come up with 20 possible options, but they all contain significant risk to the British economy.
"Leave campaigners now have to answer these key questions: what does leaving look like?; how many years would it take to put in place a new deal with the EU?; and what will happen to our security co-operation arrangements with the EU?
"Until we have answers from the Leave campaign, a vote to leave is a leap into the dark that would put the security of Britain's future at risk."
Farming minister George Eustice told BBC Radio 5 Live's Pienaar's Politics: " If I'm going to a European Council or there's a contentious issue emerging from Europe, or we're being threatened with infraction there and there needs to be a decision as to whether we challenge that or acquiesce to it - and this is the sort of bread and butter decisions that ministers like me and a department like Defra have to take - absolutely we have to have 100% access to those sorts of documents.
"And it would be very dangerous ... for the Civil Service to get into a position of saying there's a type of minister who can't have access to this information.
"Otherwise, what you're really saying is Eurosceptic ministers can't be part of the Government in future, and then that's a very dangerous scenario to get into."