Government divisions on the European Union deepened as Eurosceptic ministers poured scorn on claims that a vote to leave could create a "decade or more of uncertainty".
The first official Whitehall analysis of the process of withdrawal, published by Foreign Secretary Philip Hammond, warned 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 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 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.
"What possible evidence is there that it would take 10 years to sort out our trading arrangements?
"If you look at our relations with the European Union, 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."
The Government paper warned that the "complexity" in agreeing terms of withdrawal and setting up trade deals and other new arrangements would affect financial markets, the pound and the rights of two million expats.
"It would begin a period of uncertainty, of unknown length, and an unpredictable outcome," it said.
"The process of withdrawing from the EU is untested, and would contain numerous elements, including the outcome of the negotiations, about which there is currently little clarity.
"It could lead to up to a decade or more of uncertainty. It is important that the risk this presents is understood."
Cabinet Office minister Matt Hancock denied accusations the Government was stoking fears about the consequences of a vote to leave in the referendum on June 23, 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."
However 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."