Germany's former president Horst Koehler, who served as head of state from 2004 to 2010, died on Saturday aged 81 after a brief illness, officials announced. 

"With the death of Horst Koehler, we have lost a highly esteemed and extremely popular person who achieved great things — for our country and in the world," President Frank-Walter Steinmeier said in a letter to Koehler's widow.

Koehler, an economist by training, was the first German president who was not a career politician. Before becoming president he was the head of the International Monetary Fund in Washington.

He became German president, a mostly ceremonial role, after being put forward by then opposition leader Angela Merkel, who went on to become chancellor.

He was elected for a second term in 2009 but in May the following year resigned after being criticised over remarks about overseas German military missions. 

He died in Berlin early Saturday after a short, serious illness surrounded by his family, the presidency said.