Brand owner F W Langguth Erben said the handover process was starting "with immediate effect".
The change comes as part of a strategic review for both Langguth and Ehrmanns, coinciding with the end of the current contract period.
Langguth export director Armin Wagner said: “We are very appreciative of Ehrmanns' efforts throughout the time we have worked with them, during which there has been an excellent relationship.
"We now look forward to working closely with the Bottle Green team and using their experience and expertise to continue developments in place for both brands, such as the recent launch of Blue Nun Medium.”
Ehrmanns managing director Mark Chapman added: “In the current market conditions, we are trying to focus on countries where we feel we have specific strengths and can develop the opportunities that exist.
"We played a key role in the relaunch and repositioning of Blue Nun leading to it subsequently generating an even higher volume than its previous heyday of the seventies, and have enjoyed working with this iconic brand."
Langguth sales director Len Granger is trying to "coordinate a smooth transition to avoid any disruption to existing or new customers", the company said.