A British vote to leave the European Union could cost the economy 100 billion pounds ($145 billion) and 950,000 jobs by 2020, according to research commissioned by employers' group the Confederation of British Industry (CBI).
The CBI said "Brexit" would deliver a serious shock to the British economy, regardless of any trade deals the country could negotiate with its former European partners.
"This analysis shows very clearly why leaving the European Union would be a real blow for living standards, jobs and growth," CBI Director-General Carolyn Fairbairn said in a statement on Monday.
"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."
The CBI, which has said it will promote the economic case for Britain to remain in the EU, has been criticized by anti-EU campaigners who say the business community is split on the issue.
Two hecklers interrupted a speech by Prime Minister David Cameron at a CBI conference in November, when they unfurled a banner reading "CBI - Voice of Brussels."
In its defence, the CBI last week published research showing that 80 percent of its members wanted to stay in the European Union, while just 5 percent thought leaving the bloc would help them.
Britons will vote in an in-out referendum on June 23 and are more equally divided than business, opinion polls show, with about 40 percent on either side and about 20 percent undecided.
The director general of another employers group, the British Chamber of Commerce, resigned this month after he went public with his anti-EU views, breaching the neutral position adopted by his organisation.
The CBI, which mainly represents larger British businesses, commissioned accountants PwC to examine two different exit scenarios based on the likelihood of reaching new trade deals.
Under both, it said British living standards, economic growth and employment would be significantly reduced compared with staying.
Economic output could be curtailed by as much as about 5 percent of GDP by 2020, or 100 billion pounds, it said, while even in a scenario where a free trade agreement with the EU was rapidly secured, GDP might still be reduced by 3 percent.