Legal authorities in Ukraine say President Viktor Yushchenko was deliberately poisoned during the presidential election campaign in 2004.
The prosecutor general's office said tests had shown the presence of a toxic chemical, dioxin, in his blood.
Mr Yushchenko was seriously disfigured after falling ill following a dinner with officials of the Ukrainian security services in September 2004.
The security service insisted it had nothing to do with the poisoning.
Mr Yushchenko fell ill in the run-up to the bitterly fought election which led to his victory in the Orange Revolution.
Scientists in Vienna said tests showed the illness and facial disfigurement suffered by the opposition leader was caused by pure TCDD - a harmful dioxin.
The official investigation in Ukraine has since had further tests carried out by Ukrainian, US, German and Japanese experts and come to a similar conclusion.
The tests "allow us to think that Viktor Yushchenko was poisoned in a premeditated fashion," the prosecutor general said in a statement.
It did not give details of the supposed timing or circumstances of the poisoning, as a criminal investigation is still under way.
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