UKRAINIAN prosecutors are reopening a criminal investigation into the poisoning of Viktor Yushchenko, the opposition leader, after doctors confirmed that his disfiguring illness was caused by dioxin.

The inquiry is expected to focus on allegations that the chemical contaminant was given to Mr Yushchenko, possibly in his soup, at a dinner with top officials from the Ukrainian Security Service, the SBU, the night before he fell sick in September.

It will also address troubling questions about whether the poisoning was ordered by his rival, Viktor Yanukovych, the Prime Minister, or even by the Kremlin, which fears that Mr Yushchenko will pull Ukraine out of its strategic orbit if he wins a presidential election.

Mr Yushchenko checked out of the Rudolfinerhaus clinic in Vienna yesterday to resume campaigning for a rerun of the presidential runoff with Mr Yanukovych on December 26, which the opposition leader is now widely expected to win.

“I don’t want this factor to influence the election in some way — either as a plus or a minus,” he said, his once handsome features covered in cysts and lesions. “This question will require a great deal of time and serious investigation. Let us do it after the election.”

Earlier he thanked his doctors and said that he was “very happy to be alive here in this world today”. Dr Michael Zimpfer, head of the Rudolfinerhaus clinic, told The Times that specialists would carry out further tests this week to find out what kind of dioxin was used.

While dioxins from waste incineration and manufacture of paper and herbicides are commonly found in the environment at low levels, other types have been developed in specialist laboratories for use as poison, he said.

Dioxin was a component of Agent Orange, the infamous defoliant used by the United States in the Vietnam War.

Dr Zimpfer said that the dioxin found in Mr Yushchenko’s blood at more than 1,000 times natural levels was similar to that in Agent Orange but more concentrated.

“This was highly purified — it would only take a couple of milligrams,” he said, adding that it could have been put in a creamy soup, as dioxin is easily absorbed in fat. “A bit of whipped cream would have been wonderful,” he said.

Russia’s top toxicologist dismissed the Austrian doctors’ findings. “Dioxin is not a poison with an immediate effect. Its toxicity builds up over years, dozens of years, and it is impossible to receive a dose one day that would poison you the next,” Yuri Ostapenko, head of the Russian Health Ministry’s poison centre, told the radio station Ekho Moskvy.

But Dr Zimpfer said that Mr Yushchenko’s was a rare case in which the poison was taken orally and in one large dose, causing severe damage to the intestines, pancreas and liver.

Mr Yushchenko says that he became sick after dining on September 5 with the head of the Ukrainian Security Service, Ihor Smeshko, and his deputy, Volodymyr Stasyuk, at the latter’s country house to discuss the election campaign.

Mr Yushchenko’s wife, Kateryna, has claimed that she tasted something metallic or medicinal on her husband’s lips when she kissed him that evening. There was no immediate comment from the outgoing President, Leonid Kuchma, who backed Mr Yanukovych. Taras Chornovyl, Mr Yanukovych’s campaign manager, said that there was “no logic” in suggestions that the Prime Minister had been involved.

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