"One thing people are afraid of is that the pound would strengthen even further – that would make it more expensive [for companies in the eurozone] to invest in Britain," Andreas Meyer-Schwickerath, Director of the British Chamber of Commerce in Germany (BCCG), told The Local.
Meanwhile, just over 29 percent told the Bertelsmann Foundation that it would negatively impact jobs in their company.
Overall, 29 percent of firms said that they would reduce or relocate their capacity in the UK if the country were to leave.
But among IT firms, that figure jumped to 41 percent – while the number who would increase capacity in the event of a Brexit stood at just 16 percent.
"The prospect that almost a third of British and German companies threaten to reduce or remove their activities in the UK should cause concern among politicians as well as the general public," the authors wrote.
Overall, 79 percent of the managers surveyed thought Britain should remain a member of the European Union, with 76 percent of British firms and 83 percent of German firms agreeing.
Brits less confident
Around half the senior businessmen in the Bertelsmann survey thought that a Brexit would be "neutral" in its impact on turnover, investment and jobs.
But pessimism was stronger among Brits than Germans, with 40 percent of UK managers expecting a hit to their company's turnover compared with just 32 percent of Germans.
Looking beyond their individual comapanies, managers' fears were more widespread.
A combined 42 percent of the managers said that the effects of Brexit would be "very negative" or "relatively negative" for employment in the first three years, compared with 13 percent who expected it to be positive.
British companies were again less confident than Germans, with 44 percent expecting a negative impact compared with 39 percent of Germans.
Roughly 42 percent responded that the effects would be neutral.
"We assumed that business circles, unlike normal British citizens at the moment, would react with greater concern to the possible decision of the UK to leave the EU," the study authors wrote.
"Our survey confirms this expectation... the voices that predicted negative consequences of Brexit outweighed the positive forecasts significantly every time."
Firms love internal market
Asked what the biggest advantage of EU membership was for their business, 407 companies – 52 percent – said that the bigger internal market topped the list.
The authors said that they had presented firms with a scenario that imagined the UK gaining access to the EU internal market on the same favourable terms as Switzerland.
"If Brexit were only to mean even partial restrictions in access to the many advantages of the EU internal market, one could reckon with even more negative predictions and a significant decrease in neutral forecasts," the authors wrote.
Business' second preference came with the 22 percent who prized the EU's larger labour market.
As for the EU's negative aspects, around one-third said that complex regulations hindered their business and 22 percent complained about fears for the future of the Euro single currency.
"Most people are saying there's a lot of red tape which needs to be cut down," Meyer-Schwickerath said. "Not everything needs to be regulated."
But 146 businesses – 19 percent – said that they had no problems due to their country being a member of the European Union.
The Economist Intelligence Unit surveyed managers at 782 companies divided roughly 50-50 between Britain and Germany on behalf of the Bertelsmann Foundation.
Of the companies surveyed, 362 had annual turnovers of between €10 million and €500 million, and 420 with a turnover of more than €500 million.
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