Clothing, transport equipment, fruit and vegetables, fish and chemicals are among the industries that would be worst hit by losing trade deals with 50 countries around the world, the study said.
The report, by the Centre for Economics and Business Research (CEBR), looked at the value of goods imported in 2014, and compared existing tariffs with those the UK would be likely to face under 'most favoured nation' status with the World Trade Organisation.
UK importers trading with Turkey and Norway would stand to lose the most, as the cost of imports is expected to rise by £407 million and £109m respectively.
The competitiveness of exporters would also be damaged as their customers would face a tariff bill of almost £700m on the goods they currently buy from the UK, the CEBR said.
"The £500m difference between the impact on imports and exports shows that UK businesses have more to lose than those in the countries we will have to renegotiate new deals with," Open Britain said.
"The UK would be better placed to negotiate to keep its current access to EU free trade agreements if it chose to remain a member of the single market and the customs union after leaving," the group said.
Lord Mandelson, former EU trade commissioner, said: "The billion-pound bill for losing access to trade deals with over fifty countries would be footed by businesses and passed on to consumers with higher prices in the shops. Leave campaigners talk about all the free trade deals we can sign outside the EU but do not appreciate the value of those we already have. The EU is a leader in global free trade and we should seek to preserve its benefits, as far as is possible."