Leaving the European Union would prompt "up to a decade or more of uncertainty" for Britain, Whitehall officials have concluded in the Government's first official analysis.
The paper - seen by The Guardian - warns that the "complex" process of agreeing terms of withdrawal and setting up trade deals and other new arrangements would hit financial markets, the pound and the right of two million expats, the newspaper said.
Its release will further fuel accusations by pro-Brexit campaigners that David Cameron and allies are resorting to "fear" tactics to secure a "remain" vote in June's referendum.
London mayor Boris Johnson accused them of making "a series of questionable assertions" about the impact on the economy and security.
It comes with pressure mounting on the head of the civil service to answer concerns over a ban on ministers who have broken ranks to push for a divorce from Brussels being shown some official papers.
Cabinet Secretary Sir Jeremy Heywood will be grilled by MPs on Tuesday about the edict and ministers were braced for a possible urgent question in the Commons.
Work and Pensions Secretary Iain Duncan Smith, one of five Cabinet ministers backing "leave", said he and Eurosceptic colleagues "must have the right to continue to look" at material as he was "constitutionally" in charge of his department.
Downing Street insisted the restrictions applied onlyto areas directly related to the referendum and had been unanimously approved by the whole Cabinet as part of an agreement allowing them to remain in the Government while opposing its official policy.
The Government's analysis, drawn up by Cabinet Office officials, was reported to conclude 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.
Work on new trade deals with some of the 50-plus countries that have arrangements with the EU would be "constrained" while the process went on, it was reported to say.
Among issues to be resolved would be health insurance, cross-border security, fishing rights and access to the agency that monitors the safety of medicines.
Cabinet Office minister Matt Hancock said: "This government analysis shows that leaving the EU would lead to a decade of damaging uncertainty. The risks to our economy are clear and would leave the jobs and prosperity of the British people dangerously exposed."
Mr Johnson, writing in his Daily Telegraph column, said: "It is now obvious that the Remain campaign is intended to provoke only one emotion in the breast of the British public and that is fear."
They hoped, he wrote, that voters would "continue to sit trapped like passengers in the back seat of some errant minicab with a driver who cannot speak English and who is taking us remorselessly and expensively in the wrong direction".
He accused Chancellor George Osborne and the Treasury of "talking up" threats to the economy by persuading G20 finance ministers to include a dramatic warning that Brexit would cause a "shock" to the global economy in a post-summit communique.
"Surely the first time any country has used an international forum actively to talk up threats to its own economic prospects," he wrote.
A British official said concerns had been raised well in advance of the summit by the US and China among others and dismissed the idea such countries could be told what to say as "ridiculous".
Conservative divisions on the issue were starkly illustrated at the weekend when Mr Duncan Smith accused Mr Cameron of showing "a low opinion of the British people" by downplaying the UK's prospects outside the EU.
He insisted a favourable trade deal with the rest of the EU was "very do-able".
But Mr Cameron, who is embarking on the latest leg of a tour of question sessions around the UK, renewed his charge that opponents of continued membership were offering only "vague" ideas of how Britain would prosper outside the EU.
Writing in the first edition of the New Day newspaper, he said: "They tell you the grass would be greener - but they can't or won't say how.
"All that arises from their case is a string of unanswered questions. The only certainty is that their plan to take us out of Europe could lead to a decade or more of uncertainty.
"The choice is clear: between a greater Britain and the great unknown. I hope readers will choose certainty and prosperity over speculation and risk. Then we can carry on making this great country greater still."