A UK exit from the EU would cause a "serious economic shock", potentially costing the country 100 billion pounds and nearly one million jobs, according to a report commissioned by the Confederation of British Industry (CBI).
The business lobby group said a study found that a vote to leave would have "negative echoes" lasting many years, according to the (BBC).
It said the cost could be as much as 5% of GDP and 950,000 jobs by 2020.
But Vote Leave chief executive Matthew Elliott said employment and the economy would continue to grow after an exit.
He said that "even in the CBI's skewed choice of scenarios for exit" it was "forced to admit" that would happen.
CBI director general Carolyn Fairbairn said an EU exit "would be a real blow for living standards, jobs and growth".
She said: "The savings from reduced EU budget contributions and regulation are greatly outweighed by the negative impact on trade and investment.
"Even in the best case this would cause a serious shock to the UK economy." For the CBI, accountancy firm PwC examined what would happen if Britain signed a free trade agreement with the EU, or decided to conduct business as a member of the World Trade Organisation, in the event of the UK voting to leave in the referendum on 23 June.