A Brexit would lead to a “decade of uncertainty” because of the time it would take for Britain to negotiate new trading relationships, according to an official government paper.

The report was denounced by Eurosceptics as being part of a “Project Fear” offensive by the government, although Leave campaigners have yet to explain how the UK would conduct trade in the event of an Out vote.

The official analysis by the Cabinet Office says the uncertainty would hit “financial markets, investment and the value of the pound” as Britain negotiated new trading ties with the EU and other countries.

It also claims that the rights of British expat workers could be put in jeopardy, including access to pensions and healthcare.

“A vote to leave the EU would be the start, not the end, of a process,” the report, written by civil servants, says. “It could lead to up to a decade or more of uncertainty.”

The claim is based on the time the government believes it would take for Britain to set up new trading relationships with the EU and then to renegotiate terms with countries with which the EU already has trade deals.

Boris Johnson, London mayor, has accused David Cameron and other Remain campaigners of trying to “spook” voters by coming up with scare stories about life outside the EU.

“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,” he wrote in his Daily Telegraph column.

“They want us to go to the polls in such a state of quivering apprehension that we do the bidding of the Euro-elites, and vote to stay in the European Union. We may accept, intellectually, that the system is unreformed, and often corrupt, and increasingly anti-democratic.”

Leader of the House of Commons Chris Grayling, said: “People will not be impressed with this relentless campaign of fear. Claims that it will take twice as long to sort out a free trade deal with the EU as it did to win World War Two are clearly ludicrous. There’s a free-trade zone from Iceland to the Russian border and Britain will still be part of it after we Vote Leave.

Claims that it will take twice as long to sort out a free trade deal with the EU as it did to win World War Two are clearly ludicrous

“The In campaign claims we have no real choice other than giving away more power and money to the EU every year. It isn’t true. The real uncertainty is voting to stay in an EU which is already struggling with the Euro crisis, the migration crisis, and a youth unemployment crisis. It is safer to take back control and spend our money on our priorities.’