Mr Koehler, whose job is largely ceremonial, had linked missions such as the Afghanistan deployment with the defence of economic interests.

His remarks drew criticism from a number of German politicians.

Mr Koehler, 67, was re-elected last year to serve a second five-year term as president.

He made the controversial remarks in a radio interview after a brief visit to Afghanistan earlier this month.

He said that for an export-orientated country like Germany, it was sometimes necessary to deploy troops "to protect our interests... for example free trade routes".

Announcing his resignation on Monday, he said "it was an honour for me to serve Germany as president".