Full access to Europe's single market is vital for UK businesses and jobs, a group campaigning for Britain to remain in the EU has said.
The chair of Britain Stronger In Europe, Stuart Rose, said leaving that market would be a "huge risk".
However, "out" campaigners accused him of "scaremongering" and said he ignored the costs of being in the EU.
Out campaigners also claimed support from new research by the independent think tank Civitas.
It says membership of the single market has not had a significant impact on export growth.
The prime minister, who wants the UK to stay within a reformed European Union, is pushing to renegotiate Britain's terms of membership ahead of an in/out referendum, which must be held by the end of 2017.
If agreement with other EU leaders is reached next month, a vote could potentially be held as early as June.
Lord Rose told the BBC that campaigns to leave the EU had not explained how the benefits of the EU single market would be replaced.
If Britain votes to leave, "it's a huge risk, we're taking a huge risk", he told BBC Radio 4's Today programme.
About 50% of UK exports are to Europe whereas BRICS countries - Brazil, Russia, India, China and South Africa - accounted for 8%, he said.
Lord Rose presented his campaign's case at the plant of Britain's biggest bike manufacturer, Brompton Bicycle, in west London.
He said: "Those who want Britain to leave Europe cannot guarantee that Britain will retain full access to Europe's single market. They are putting the benefits at risk. Their proposed deal, whereby Britain would somehow retain access to the single market without obeying any of the rules, is a fantasy."
In response, Robert Oxley from the Vote Leave campaign said: "I think this is just further scaremongering from the Britain Stronger in Europe campaign which ignores the cost of the EU."
Britain Stronger in Europe said one piece of research showed UK goods trade with the EU was 55% higher because of EU membership.
Their claim is based on research by the Centre for European Reform that was first published in January 2014.
The research used a statistical model to estimate how much extra trading of goods the UK does with other EU members, than would be expected without the single market.
Vote Leave said it was "out of date research from a pro-EU think tank".
Vote Leave used research from Civitas to support its case.
Civitas studied official trade statistics and said that Britain had recorded slower export growth than any of the other founding nations of Europe's single market.
Michael Burrage, who wrote the report, said: "While the single market cannot be counted a success in export terms for the EU as a whole, for the UK it must be counted at the very least a massive disappointment, and not far short of a disaster."
His report found growth in UK exports had tended to drop after European Commission trade deals, whereas independent countries Switzerland, Singapore and South Korea increased exports in the majority of cases after they negotiated their own trade agreements.
Mr Burrage added that UK export growth was 22.3% lower since joining the single market at the end of 1992 than it would have been had it continued at its rate during the common market between 1973 and 1992.
Vote Leave chief executive Matthew Elliott said: "The unquestioning mantra that the single market has been good for British trade is wrong and should be challenged as this research makes crystal clear."
Meanwhile, pharmaceutical executives warned that a British exit from the EU could isolate the country's scientists and reduce its influence in medicine.
John Lechleiter, chief executive of the US pharmaceutical company Eli Lilly, told the Financial Times it would be a "shame and a mistake" if the UK left the EU, adding a vote to leave would "isolate the UK, to its detriment".
The boss of Unilever, Paul Polman, said the consumer goods giant would not scale back its 7,500 UK jobs in the case of an "out" vote, but he did feel the UK had "got a lot of benefit from the union".
Mr Polman told the Guardian it would be "very good" if Britain voted to stay in the EU.
David Cameron last week urged business leaders who want Britain to remain in a reformed EU not to "hold back".
David Cameron's four main aims for renegotiation
- Economic governance: Securing an explicit recognition that the euro is not the only currency of the European Union, to ensure countries outside the eurozone are not disadvantaged. The UK wants safeguards that it will not have to contribute to eurozone bailouts
- Competitiveness: Setting a target for the reduction of the "burden" of excessive regulation and extending the single market
- Immigration: Restricting access to in-work and out-of-work benefits to EU migrants. Specifically, ministers want to stop those coming to the UK from claiming certain benefits until they have been resident for four years
- Sovereignty: Allowing Britain to opt out from further political integration. Giving greater powers to national parliaments to block EU legislation
Referendum timeline: What will happen when?