David Cameron has rejected accusations that the Remain camp is engaged in a "Project Fear" scaremongering campaign in the EU referendum, insisting that he is interested only in "Project Fact".
But his assurance was dismissed as "baloney" by London mayor Boris Johnson, in a further indication of the depth of splits at the top of the Conservative Party over the vote on June 23.
Allegations of scaremongering were fuelled by the publication of a new analysis which suggested a vote to leave could create a "decade or more of uncertainty" for exporters, financial markets and two million expatriates.
The first official Whitehall analysis of the process of withdrawal, published by Foreign Secretary Philip Hammond, warned that an "out" vote would mark the start of "a period of uncertainty, of unknown length, and (with) an unpredictable outcome".
The 23-page document said it was unlikely the terms of withdrawal and future trade arrangements could be fully negotiated within the formal two-year process, opening the door to other EU states demanding concessions in return for an extension.
But Brexit-backing Leader of the Commons Chris Grayling dismissed the findings, insisting it would be just as much in the interests of the remaining 27 nations to reach a speedy conclusion to the negotiations.
"Why on earth would we think it would take twice as long as the Second World War to be able to sort out our trading relationships with Europe and elsewhere?" he said on BBC Radio 4's Today programme.
"We have a £50 billion-plus trade deficit with the European Union. They sell far more to us than we sell to them, they lose out - their jobs, their businesses are in danger if we do not sort it out quickly."
Speaking to students in Ipswich, Mr Cameron rejected the "Project Fear" tag attached to the Remain campaign by supporters of UK withdrawal.
"The only project I'm interested in is Project Fact," he said. "Project Fact is about saying, 'Stay in and you know what you'll get'.
"This is not about raising concerns and worries which aren't there - they are real concerns and worries based on fact."
Mr Johnson told LBC radio during a visit to Northern Ireland: "Project Fear is going into overdrive. They are trying to scare the pants off everybody at the moment, and it's not working.
"I think everybody can see that there's a huge chance to make a change and have a better relationship and a more honest relationship with the EU."
Scottish First Minister Nicola Sturgeon - who is backing the "in" campaign - cautioned ministers against relying on fear tactics, saying they had backfired when they were adopted by the No campaign in the Scottish independence referendum.
"One of the undoubted lessons of the Scottish experience is that a miserable, negative, fear-based campaign saw the No campaign in the Scottish referendum lose over the course of the campaign a 20-point lead," she said in a speech in London.
"I don't have to point out to anybody here that the 'in' campaign in this referendum doesn't have a 20-point lead to squander."
Cabinet Office minister Matt Hancock denied accusations the Government was stoking fears about the consequences of a vote to leave, insisting the analysis was a "cautious assessment".
"The truth is we don't know. Those who are proposing to leave, I think it's incumbent on them to have to explain exactly what would happen," he told the Today programme.
"What would happen to the two million Brits who live in other places in the European Union? Would they still have access to free healthcare? There are real consequences of this for jobs and for livelihoods."
Mr Hancock was forced to come to the House of Commons to answer growing anger among Eurosceptics over a ruling by Cabinet Secretary Sir Jeremy Heywood that ministers backing the "out" campaign will not receive official papers relating to the referendum.
Employment minister Priti Patel said Sir Jeremy's action was "unconstitutional" and accused him of jeopardising the political impartiality of the Civil Service.
"Secretaries of state are responsible for their departments. For an unelected official to prevent them being aware of the information they need for their duties is wrong," she said.
Influential backbencher Bernard Jenkin suggested the move could conflict with the Civil Service's "duty of honesty".
"Nobody objects to the Government making its case in this referendum but most people expect the Civil Service to be impartial in carrying out its support for ministers," said Mr Jenkin, chairman of the Commons Public Administration and Constitutional Affairs Committee.
"It is established in law that ministers are accountable for their departments and voters expect Government facts and figures to be impartial and accurate, whether they are used by ministers who support Remain or Leave."
Labour deputy leader Tom Watson said Mr Cameron had "a simple choice - either he gives his ministers free rein to run their departments, or he sacks them. It cannot be fudged for the next 114 days".
But Mr Hancock said the arrangement was "the best way to manage the unusual situation of ministers who disagree with the Government remaining in post". He stressed that the Civil Service code makes it clear that the "duty" of civil servants is to "support the position of the Government of the day".