Access to the European Union single market has failed to benefit UK trade, analysis by a think-tank has suggested.
The study indicated that pro-European claims that the British economy gained a trade advantage by being a member of the 28-state bloc was "empty rhetoric".
The report by centre-right think-tank Civitas said Britain's trade with other EU nations accounts for no more of its trade with all leading economies than it did on joining the European Economic Community in 1973.
Meanwhile, exports to non-EU nations Iceland, Norway and Switzerland have increased enormously over the same period.
The report analysed and compared figures since 1960 and concluded that there was no "insider advantage" to being within the EU in terms of exports.
Report author Michael Burrage, a director of Asian market research firm Cimigo, said: " Insider advantages will only rate inclusion in any serious assessment of the economic case for EU membership when those who make claims about them identify and document specific advantages for UK exports.
"Until they do so, we may reasonably conclude that they cannot form a sensible basis of public policy decisions and do not contribute to an intelligent debate about the merits of EU membership.
"That debate has to recognise that, thus far, the single market has not enabled UK exports of goods or services to other members to grow at a faster rate than those of non-member exporters, nor at a faster rate than UK exports to non-member markets.
"It has been an era of decline for UK exporters, relative to both non-members in the same market, and to UK exports to other markets."
The report found that UK goods exports to the other 14 pre-2004 EU nations have not increased as a proportion of its trade with OECD nations since Britain joined the European Economic Community in 1973.
The proportion of UK goods exported to non-EU Iceland, Norway and Switzerland has doubled from 5.1% in 1973 to 10.7% in 2012. Services exports to these nations trebled from 6.1% to 20.2% over the same period.
The Civitas study also cast doubt on pro-European claims that Britain had more clout to negotiate trade agreements as part of the EU.
There were 25 EU free-trade agreements in force in 2012 while non-EU Switzerland had independently negotiated 26, the report said.