A leading British employers group stepped up its push to keep the country in the European Union on Wednesday, saying the alternatives would hurt trade.
As campaigning intensifies before a referendum on remaining an EU member, the Confederation of British Industry listed concerns among a sample of 29 member companies about leaving.
"Whilst it's not a uniform view, the majority of firms believe that the 'pros' of EU membership outweigh the 'cons,'" the CBI said in a report. "But they also recognise that, like most institutions, the EU is far from perfect."
The CBI, which represents 190,000 firms, backs British Prime Minister David Cameron's efforts to secure changes to Britain's relationship with the EU before the referendum. Cameron has promised a vote by the end of 2017; expectations are it will take place next year.
Dominic Cummings, campaign director of Vote Leave, one of the two campaigns pushing for Britain to leave the EU, said the CBI leadership did not represent business opinion and had been wrong on previous decisions concerning the EU.
"It's campaigning to stay in the EU regardless of the terms," Cummings said. "It has zero credibility on the EU."
"It campaigned for Britain to join the ERM (exchange rate mechanism) which was a disaster. It campaigned for Britain to scrap the pound which would have been a disaster," Cummings said. "They were wrong then and they are wrong now."
Campaign group Leave.eu also dismissed the CBI's stance.
"If the CBI wants to stay in a reformed EU, then what will the CBI do if none of the reforms are achieved? It's crystal clear to everyone that Cameron will not achieve any reforms," said Richard Tice, one of Leave.eu's business ambassadors.
But the CBI said the kind of arrangements that give access to the EU market to some countries outside it failed to offer a better package than membership for Britain.
Switzerland took nine years to negotiate its first trade deals with the EU and only gained access to some parts of the single market, with no formal influence over the rules it must comply with, the CBI said.
A Norway-style arrangement would mean Britain still had to follow the bloc's rules, including those allowing workers from EU countries to come to the country, something many British critics of the EU object to.
A special EU-Britain trade deal - another idea supported by British opponents of EU membership - would put trade at risk and reduce Britain's influence over its terms, the CBI said.
A CBI survey carried out in 2013 found 80 per cent of its members supported staying in a reformed EU.
A newly launched group campaigning for Britain to stay in the EU said the report "blows a huge hole" in the arguments of rival campaigners.
"The CBI's report fundamentally undermines their extreme view that Britain has nothing to lose by leaving Europe," the executive director of Britain Stronger in Europe, Will Straw, said in a statement.