By Christopher Alessi and William Wilkes 

Most German companies are unprepared for a potential British exit from the European Union, according to a leading German industrial group.

Almost 70% of firms surveyed by the Federation of German Industries, or BDI, "don't know how they would react to Brexit," said BDI Chief Executive Markus Kerber in a German radio interview on Wednesday.

If Britons vote to leave the bloc in a referendum Thursday, the implications could take months or years to become clear. For German companies and others in the EU, the prospect is so unprecedented that they lack a baseline from which to work.

"We haven't been able to make many contingency plans," Juergen Maier, chief executive of UK operations for German industrial conglomerate Siemens AG, said in an interview. "We don't have a clue what actually might happen" regarding tariffs, regulations and labor rules, he said.

Trade between Germany and the U.K. is vital to both countries. Germany was the U.K's second-largest export market after the U.S, according to Britain's national statistics agency. The U.K. was Germany's third-largest export market last year, behind the U.S. and France, according to the German Federal Statistical Office.

Alexander Moscho, the chief executive for U.K. and Ireland at German pharmaceuticals and chemicals group Bayer AG, said in an interview that a major concern for Bayer would be how new regulations could affect licensing frameworks for medications, potentially limiting patients' access to needed medicines in the future.

"All of us need to exactly understand what it means" if Britons vote to withdraw, he said.

Mr. Moscho said a big problems was lack of clarity on what kind of relationship post-exit Britain would seek with the EU regarding trade, commerce and the freedom of movement across borders. He cited both Norway and Switzerland as different potential models of European nations not in the EU that maintain strong ties with the bloc.

Germany's chemical industry, which maintains a trade volume of roughly EUR18.5 billion with the U.K., has vocally warned of pitfalls in a Brexit.

The biggest risk facing the industry would be weaker British demand for its chemicals, said Dr. Henrik Meincke, the chief economist at the German Chemical Association, or VCI. "We expect the U.K., in the case of a Brexit, to slip into recession," Dr. Meincke said.

"I can't see any benefits for the U.K. or Germany if Brexit happens," said the BDI's Mr. Kerber. Later Wednesday, he told the BBC that in the event of a Brexit it would be "very, very foolish" for the U.K. or EU to impose trade barriers.

Proponents of a British withdrawal from the EU immediately seized on those comments as evidence that a Brexit would not harm the U.K.'s economic ties with the EU.

"Of course EU countries will continue trading with us on a tariff-free basis -- they would be damaging their own commercial interests if they didn't," former London Mayor Boris Johnson said Wednesday while campaigning for Brexit.

A BDI spokesman declined to comment on the reaction to Mr. Kerber's comments. BDI's official position supports Britain remaining in the EU.

Write to Christopher Alessi at christopher.alessi@wsj.com and William Wilkes at william.wilkes@wsj.com

(END) Dow Jones Newswires

June 22, 2016 11:45 ET (15:45 GMT)