According to the Bertelsmann Stiftung, the implications of a Brexit "are clear", with its CEO Aart De Geus saying, "In the case of a Brexit, everyone would lose."
A study by the German think tank concludes that the economic costs to the UK of quitting the EU after the referendum on 23 June would be "enormous."
"And the EU's international position would be weakened as well. Germany would suffer negative economic repercussions and would lose an important political ally in Europe," De Geus says.
On the potential fallout should Britain leave the EU, Bertelsmann Stiftung examined a number of scenarios.
In the most extreme case, it says gross domestic product in the UK would shrink by up to €313bn over the long term and GDP in Germany would decline by up to €58bn.
A survey by the same think tank showed that business leaders in both Germany and the UK view a Brexit as a "threat."
Bertelsmann Stiftung says, "Bureaucratic, undemocratic and out of touch are some of the persistent clichés used to describe the EU, and not just in Britain.
"Yet the image people actually have of the EU is more complex than often thought."
It points out that according to a recent EU-wide survey, most people still trust in the euro and would like to see more integration among member states.
Findings also show, however, that people in the UK are least informed about the EU, and the desire to leave the EU is most pronounced there, says Bertelsmann Stiftung.
Meanwhile, Leave campaigners have pledged to set up an Australian-style immigration points system by 2020.
In a statement, Vote Leave campaigners Michael Gove, Boris Johnson, Priti Patel and Gisela Stuart said, "We want Britain to continue to benefit from migration.
"But if we are to welcome more people to Britain then the public must be reassured that we have control over who comes here."
In a series of pledges, they promise that "there will be no change for Irish citizens. The right of Irish citizens to enter, reside and work in the UK is already enshrined in our law," that, "there will be no change for EU citizens already lawfully resident in the UK," and that, "by the next general election, we will create a genuine Australian-style points based immigration system. We could then create fairness between EU citizens and others."
They conclude, "If we implement these principles, for the first time in a generation it will be possible for politicians to keep their promises on migration."