By Andrew Osborn
LONDON (Reuters) - Britain's membership of the European Union boosts its trade in goods by about 30 percent and there is no evidence it impedes trade with countries outside the 28-nation bloc, a pro-European think tank said in a report on Monday.
The London-based Centre for European Reform (CER) predicted Britain would struggle to maintain trade with other EU member states - now 54 percent of goods trade - if it left the bloc.
Prime Minister David Cameron has promised to renegotiate Britain's EU ties if his Conservative party is re-elected in 2015, and then give Britons an in/out membership referendum.
Fearing defeat at the hands of the anti-EU UK Independence Party (UKIP) at European Parliament elections in May, lawmakers in his own party want him to explain by the end of February how he would renegotiate.
Polls show a slim majority of Britons are fed up with what they see as the EU's overbearing interference in national life and would vote to leave the bloc if given the chance.
The CER said it had used what it called "a widely-used economic model" to test whether the EU boosted Britain's trade and whether, as suggested by some EU critics, it constrained its ability to tap markets outside Europe.
It said it had concluded Britain's trade in goods with other EU member-states was 55 percent higher than would be expected given their output, proximity and other traits, and that this means membership boosts total goods trade by around 30 percent.
John Springford, one of the report's authors, said: "Any decision about leaving the EU must be based on an appraisal of what Britain would be leaving: the EU has successfully reduced trade barriers with the many rich economies on Britain's doorstep. An exit would imperil those gains."
The report said it had looked at trade agreements non-EU member states Norway and Switzerland had with the bloc to get an idea of what Britain might be able to negotiate if it left. Both scenarios meant Britain would be worse off, it said.
"Outside the EU, the UK would face a difficult dilemma: it would have to negotiate access to the EU's single market in exchange for continued adherence to its rules," said Simon Tilford, a report co-author. "British eurosceptics would like to leave the EU in order to re-establish regulatory sovereignty: but they must accept that Britain would have less access to the European markets as a result."
EU critics in the Conservative party say EU rules are too onerous and are choking small and medium-sized firms. They argue that the world's sixth largest economy would be able to broker better trade deals with emerging economies if it left the EU.
(Editing by Peter Graff)