Leaving the European Union could cause a "serious shock" to Britain's economy, with the risk of losing almost one million jobs, according to a CBI business group study released.
A so-called Brexit could cost about £100 billion (128 billion euros, $145 billion) of economic output by 2020 -- equivalent to five percent of annual GDP -- the research found.
"This analysis shows very clearly why leaving the European Union would be a real blow for living standards, jobs and growth," said Carolyn Fairbairn, director general of the CBI.
"Even in the best case this would cause a serious shock to the UK economy."
An exit could cost as many as 950,000 jobs, according to the study, meaning unemployment would be 2-3 percent higher by 2020 than if Britain remained in the EU.
The research was undertaken by services firm PwC on behalf of business group the Confederation of British Industry.
Vote Leave, which campaigns in favour of an exit, dismissed the results, with chief executive Matthew Elliott telling the BBC the CBI's scenarios were "skewed" and leaving the EU was the "only safe option".
Britain is due to vote in a referendum on whether to remain a member of the 28-country bloc on June 23.
Opinion polls indicate the race is neck and neck with as many as 20 percent of voters still undecided, meaning the result is very much in the balance.