Britain leaving the European Union could result in a permanent loss of 2.2 percent of the country's gross domestic product by 2030, and the costs would not be offset solely by striking a free trade deal with its former partners, think tank Open Europe said on Monday.
The prospect of Britain breaking away from Brussels has moved up the political agenda in tandem with a surge in support for anti-EU party U.K. Independence Party (UKIP) in recent years.
The Conservative Party, which has long contained a wing skeptical about the European Union, has promised an in-or-out referendum on membership before the end of 2017 if they win national elections on May 7 in a move designed to neutralize UKIP's appeal.