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"It's a final decision, it can't be changed in any way," the 35-year-old striker told Czech daily newspaper Mlada Fronta Dnes.
Koller is the nation's all-time leading scorer with 52 goals in 85 matches and he will have the chance to extend his record when playing in Austria and Switzerland.
The big man (2.2m) has also revealed that his decision was influenced by Karel Bruckner's announcement that he will quit as team manager after the Euro finals.
"He's one of the best coaches I've ever had," Koller said.
The German-based striker is set to end a nine-year career with the Czech national team as he made his debut in 1999 but failed to clinch any silverware.
They have been drawn in Group A in the Euro 2008 finals along with Switzerland, Turkey and Portugal.