Debates over migration should not be allowed to "dominate" the upcoming referendum on EU membership, the leader of the main campaign for Britain to remain a member of the 28-nation bloc has said.
Britain Stronger in Europe chairman Lord Rose urged voters to focus on the economic case for continued membership, as he released analysis suggesting the EU is worth an average of £670,000 in extra trade for each business that exports or imports goods within the bloc.
The former Marks & Spencer boss acknowledged the EU was "imperfect" but argued that its benefits to the UK outweigh its costs by a factor of 10 to one and warned voters that Britain faces "the risk of the unknown" if it leaves.
Lord Rose's warning came as pro-Brexit group Vote Leave seized on the findings of a study that claims the single market has had "no discernible benefit" for UK exports and has proved "not far short of a disaster" for Britain.
Research from thinktank Civitas found UK export growth in the single market area was 22.3% lower following the creation of the EU in 1993 than it would have been had it continued at its trend rate during the common market years of 1973-92.
Prime Minister David Cameron will head to Brussels next month to try to finalise renegotiations of Britain's relationship with the EU before putting the deal to the country in a referendum before the end of 2017.
Lord Rose said he hoped Mr Cameron would secure a deal in February allowing an early referendum - which some observers believe could come as soon as June.
Speaking to LBC radio, the Stronger In chairman recognised that migration was "the issue of our time", but added: "I don't want this referendum to be totally dominated by the migrant crisis."
He called on Eurosceptics to set out evidence on what future trade deals would look like outside the EU, arguing that everything from food to clothing, travel and insurance would be more expensive if the UK left.
Britain Stronger In Europe is highlighting research by the Centre for European Reform that found Britain's goods trade with the EU is 55% higher as a result of its membership.
The "EU effect" was worth around £133 billion to the 200,000 export and import companies in the UK in 2014, it said.
Lord Rose told LBC: "The EU is not perfect. I have always been a bit of a Eurosceptic. But, I say to myself, it is the reality of what we have got today versus the risk of tomorrow. What we don't know is what is the risk?
"We are not in the euro, we are not in the Schengen agreement, we haven't got open borders, we have got some protections, we are not going to go into the euro and we do get massive benefits form being in the EU.
"We have got £350-odd a year the cost of us being in, per person, and £3,000 a year the benefits - calculated independently - of being in. That's a 10-to-one return. Do we want to sacrifice that?
"I am pro-Europe in the sense that I would absolutely be in Europe, with its imperfections, rather than risk being out with the unknown."
Lord Rose said World Trade Organisation rules would inevitably mean higher taxes on trade with European neighbours if Britain left.
Vote Leave highlighted the Civitas reported which analysed trade statistics and found Britain's membership of the EU has failed to have a significant impact on export growth.
Academic Michael Burrage, who wrote the report, found the bloc has boosted the exports of non-EU countries more than its members, with Britain recording slower export growth than any of the other founding nations.
Growth of exports between member states during the common market was 4.7% but has fallen to 3% in the single market, the research said. UK export growth fell from 5.38% to 3.09% over that period.
Exports between the 12 founding members of the single market are 14.6% lower than if they had continued to grow between 1993 and 2012 as they had done under the common market, the report adds.
Mr Burrage 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."
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."