Yushchenko, who was a leader of the political opposition at the time, was poisoned with dioxin during the 2004 presidential election campaign, leaving his face disfigured. No arrests have been made, but suspicions of Russian involvement persist - both because Yushchenko was running against a Kremlin-backed candidate and because Russia is one of four countries that produces the specific formula of dioxin used against him.
Although the dioxin from all four countries is chemically identical, differences in the manufacturing process yield various byproducts; testing samples from each country for these byproducts could determine the origin of the dioxin found in Yushchenko.
Three countries that produce this type of dioxin - Britain, Canada and the United States - have submitted samples to Ukraine for testing, investigators say, but Russia has refused. This month it offered to test samples of its dioxin in Russia and report the results to Ukraine.
But the Prosecutor-General's office this week rejected that offer and again asked Russia to let the test be conducted in Ukraine, a spokesman said.
"Under Ukrainian law, the tests will only be valid if they are conducted on Ukrainian territory," said Yuriy Boychenko.
The Kremlin strongly backed Yushchenko's rival, Viktor Yanukovych, in the bitterly contested presidential election, which deepened rifts between Moscow and the West. Yanukovych was initially declared the winner. Massive street protests - dubbed the Orange Revolution - broke out, and the Supreme Court threw out the results on grounds of fraud. Yushchenko won a court-ordered repeat vote.
Yushchenko has hinted that he knows those responsible for the poisoning. While refraining from naming the alleged culprits until the probe is over, he has hinted that the poisoning could have been masterminded from outside the country.