The German army has war-gamed the break up of the European Union in study of security crises that could face the country by 2040.

Military planners in Berlin played out a scenario in which a growing number of countries follow Britain in leaving the EU, resulting in an "increasingly disorderly" world, Der Spiegel reported.

"The EU enlargement has been largely abandoned, more states have left the bloc," strategists wrote in a study cited by the magazine.