LONDON (Reuters) - Britain will vote to stay in the EU by a "substantial margin" predicted the leader of a campaign to keep the world's fifth-largest economy in the trading bloc, slating the opposing "out" movement for what he said was failure to present an alternative to membership.
Stuart Rose, chairman of the "Britain Stronger in Europe" group, was speaking as Prime Minister David Cameron tries to convince eurosceptics of the benefits of a potential new EU deal ahead of a referendum on membership possibly as early as June.
"We'll win by a substantial margin," Rose, a former chief executive of Marks & Spencer, said at a media lunch on Thursday.
"Britain's stronger, Britain's more secure, Britain will be more prosperous if we stay in Europe," he added. "That's the choice and I'm confident we will win."
Pitching the choice of the "imperfect reality" of membership against the "uncertain risk" of an exit from the EU, Rose said the groups who advocated leaving had not made clear what sort of trading relationships Britain would have instead.
"I don't think they've got a very strong case ... tell us how 'out' will look," he said.
Opinion polls suggest voters are fairly evenly split over membership of the bloc which Britain joined in 1973. The last time it held a referendum on the issue in 1975, 67 percent voted to stay in and 33 percent voted to leave.
(Reporting by Sarah Young; editing by Stephen Addison)