A possible U.K. divorce from its European Union partners would be a messy affair, leaving thousands of past, present and future investment decisions in limbo, the director general of Germany’s top industry group said. The BDI—the Federation of German Industries representing the country’s 100,000 biggest companies including Siemens AG and Allianz SE—is resolutely opposed to a “Brexit” and is concerned that those in favor of quitting the bloc may be failing to do their homework, Markus Kerbersaid. The BDI has close ties to its Confederation of British Industry, which likewise rejects an exit from the EU. A vote to leave in the June 23 referendum would spark a “tooth-and-nail fight” between the U.K. and its erstwhile partners to secure bilateral advantages in the separation, Kerber said in an interview in Berlin Thursday. “It wouldn’t be an amicable divorce,” Kerber said. And, unlike for Norway and Switzerland, “there’s no default European free-trade status in the waiting.” Kerber’s comments underscore the increasing unease felt in Germany over the business fallout of an exit. The pain could last years, damaging or completely unraveling trade and investment ties built up over decades, said Kerber. Germany is the U.K.’s biggest trading partner, while Britain is the fourth largest trading partner for the Germans. Bilateral trade added up to 92 billion euros ($101 billion) in 2014. About 2,500 German companies operate U.K. units that employ 370,000 people—Prime Minister David Cameron visited a Siemens plant in western England this month to make a speech extolling his EU deal. Meanwhile, 3,000 U.K-based companies including GKN Plc, Terra Firma Investments Ltd. and Rolls-Royce Holdings Plc operate German units, according to German government data.

‘Economic Shock’

U.K. Chancellor of the Exchequer George Osborne told BBC Television in Shanghai on Friday that an exit from the EU would cause “a profound economic shock for our country, for all of us, and I’m going to do everything I can to prevent that happening.” “I think the damage for both sides in a Brexit would be very big, but bigger for the U.K. as 50 percent of its trade is with the EU,” said Kerber. “That means 50 percent of the country’s exports navigate unsure waters from one day to the next.” Resolving post-exit legal issues including trade treaties, taxation and regulatory matters might take five to 10 years, “embroiling companies in thousands of contract revisions,” said Kerber, who’s formerly a banker and was a Finance Ministry division head under Wolfgang Schaeuble.

Exchange Merger

German media commentary that followed the announcement this week of a potential merger of Deutsche Boerse AG with London Stock Exchange Group Plc focused on the impact of a Brexit, a development that may haunt myriad future investment decisions, said Kerber. “Of course, one can’t rule out that in five or 10 years things would improve for the U.K.—it might become a super-Singapore at the gates of Europe,” said Kerber. “But politically that’s a long, long time to wait.”