London - Britain leaving the European Union could result in a permanent loss of 2.2% 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.
The prospect of Brexit - Britain breaking away from Brussels - has moved up the political agenda in tandem with a surge in support for anti-EU party UK Independence Party (UKIP) in recent years.
The Conservative Party, which has long contained a wing sceptical 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 neutralise UKIP's appeal.