"The results of analysis carried out in three different laboratories in the Netherlands and Germany agree... that it is was TCDD" or tetra-chloro-dibenzo-dioxin, Michael Zimpfer, head doctor at Vienna's Rudolfinerhaus said.

He refused to comment further, saying Yushchenko's family did not want more details revealed.

On December 11, Zimpfer had told a press conference that Yushchenko was suffering from dioxin poisoning and that this may have been intentionally done.

French toxicologists said however that they were at a loss to explain why presumed assassins would use dioxin, a chemical that is a progressive killer, as a weapon against Yushchenko.

At extremely high doses, dioxin causes liver failure and death, but at smaller doses, it causes organ damage and cancer, which can kill but usually years down the road, they said.

Dioxin also causes a severe skin disease, chloracne, which is the most visible of Yushchenko's problems, but this is usually not a fatal condition.

"If someone was trying to kill him, cyanide or bacterial toxins would have been more effective," Professor Jean-Francois Narbonne from France's University of Bordeaux said.

"Dioxin is not the only substance capable of producing such lesions," he said, referring to the severe form of acne which has disfigured Yushchenko's face.

"There is not one dioxin but 210," Dr. Jacques Descotes from the southern French city of Lyon said.

"We don't know much about most of them, except for Seveso dioxin," Descotes said, referring to the 1976 disaster in Italy, in which more than 1,200 people were poisoned by gas that seeped from a chemical factory.

Yushchenko said last Friday that he was not accusing the SBU secret services of trying to poison him despite suspicions that he ingested a massive dose of dioxin during a dinner with two of its agents.

Yushchenko had told reporters that he suspected that he swallowed the poison during a dinner he had with SBU chief Ihor Smeshko and his deputy.

"The SBU chief and his deputy were present during the last business dinner (on September 5), but... I don't say that I am directly blaming, or directly suspect these people of my poisoning," Yushchenko said.

Yushchenko fell ill on September 6, while holding a comfortable lead over his chief presidential rival Prime Minister Viktor Yanukovich in opinion polls just weeks before a first round presidential election.

A repeat election is to be held on December 26, with the pro-Western Yushchenko, 50, seen as the likely winner.