Financial markets, investment, the value of the pound and the lives of millions of British expats would be negatively affected by a Brexit, the government's official report has found.
The Cabinet Office report also said it would take 10 years for the UK to remove itself from the European Union, causing a "decade of uncertainty". That includes the time it would take to negotiate trade deals with the EU, US and other nations.
The report said two million expats would no longer be guaranteed the right to work, access pensions and healthcare in EU countries.
“A vote to leave the EU would be the start, not the end, of a process. It could lead to up to a decade or more of uncertainty," the report said.
The report said: "The UK’s withdrawal from the EU would mean unravelling all the rights and obligations – from access to the single market, to structural funds for poorer regions, to joint action on sanctions – that the UK has acquired both during our accession to the EU and over our 40-year membership. As well as negotiating its withdrawal, the UK would also want to negotiate its post-exit arrangements with the EU."
Matt Hancock, Cabinet Office minister, 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.”