David Cameron has warned that leaving the EU would hurt the Northern Irish economy
In British government analysis seen by The Guardian, Cabinet Office officials said the process of agreeing withdrawal and setting up new trade deals would hit financial markets, the pound and two million ex-pats.
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."
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.
Writing in his Telegraph column, he 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.
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."
British prime minister David Cameron has warned that Britain leaving the EU would damage the Northern Irish economy and lead to job losses in the six counties.
Speaking at the Bushmills distillery and later at a gathering of farmers in Ahoghill, Co. Antrim, he warned that the negative effects that an EU-exit would have in Northern Ireland.
"The work being done between the Assembly government and the British government, that is going well. The relationship between the Republic of Ireland, that is going well, and so I want all those things to continue.
"My side of the argument, the Remain side of the argument, part of our case is we have got economic strength – we have got political stability. Let’s build on that by staying in a reformed European Union," the British PM said.