Shared economic interests must be a priority in the Brexit negotiations, UK and German trade bodies have urged.
The British Chambers of Commerce (BCC) and the Association of German Chambers of Commerce (DIHK) said uncertainty over "business critical" issues such as workers' rights, tax and customs arrangements needed to be tackled.
DIHK said the uncertainty was affecting German firms which traded with the UK.
Most BCC members say they want "at least" a three-year transition period.
The groups called for political leaders to "build an atmosphere of mutual trust and constructive dialogue", to deliver clarity and certainty for businesses.
With the third round of Brexit negotiations getting under way on Monday, a number of critical issues were still unresolved, while there are "hundreds" of practical and technical issues which also needed to be negotiated, they said.
"There is real business appetite from both sides for a focus on practical, day-to-day business concerns, and a desire for clarity on future trading arrangements," said BCC director general Dr Adam Marshall.
"The UK and the EU must begin work on transitional arrangements, particularly on customs, so that firms on both sides of the Channel have the confidence to make investment decisions."
The UK is the third largest market for the export of German goods, while Germany is the UK's second biggest market for exports of goods and services.
German firms employ an estimated 400,000 workers in the UK, while British firms employ around 220,000 workers in Germany.
DIHK chief executive Martin Wansleben said German companies were concerned that Brexit would have "a major negative impact", with more trade barriers such as extra bureaucracy, and stricter border controls, leading to higher costs.
"The terms of exit are still completely unclear.
"Many of our members are reporting that they are already shifting investments away from the UK in anticipation of these barriers," he added.