1027 GMT December 18, 2017
"The preliminary results of the toxicological test showed that Praljak had a concentration of potassium cyanide in his blood," the Dutch prosecution said in a statement, AFP wrote.
"This has resulted in a failure of the heart, which is pointed out as the suspected cause of death."
The preliminary results were released after an autopsy was carried out on the body of the 72-year-old Bosnian Croat commander, whose shocking final act was broadcast live around the world.
On Wednesday in a last show of defiance against the International Criminal Tribunal for the former Yugoslavia (ICTY), Praljak swallowed the poison before UN judges, just moments after they upheld his 20-year jail term for war crimes.
He remains a hero to many Croats despite his conviction for crimes during Bosnia's 1990s conflict.
The Dutch prosecution service said "Praljak passed away in Westeinde hospital in The Hague after consuming a fluid substance in the courtroom".