Yurko Pavlenko, an opposition MP, said the findings by an Austrian clinic confirmed their suspicion that a mysterious illness that left their candidate’s face deeply pockmarked and discoloured had been caused by dioxin — possibly at the instigation of the Kremlin.

“This was done by people who wanted to kill him or effectively knock him out of the presidential race,” Pavlenko said. “Russia has campaigned against Yushchenko and interfered in a shameless way in the Ukrainian election, and its intelligence sources cannot be excluded from the top suspects.

“This confirmation will boost Yushchenko’s vote and will especially bring undecided voters to his side.”

Yushchenko, 50, will face Viktor Yanukovych, 54, the pro-Moscow prime minister, in the Boxing Day contest — a rerun of the second round of the election on November 21.

Yanukovych’s victory in that contest was annulled as fraudulent by the supreme court after the result brought hundreds of thousands of protesters onto the streets of the capital, Kiev, and other cities.

Dr Michael Zimpfer, director of Vienna’s private Rudolfinerhaus clinic, said extensive tests on Yushchenko had found levels of dioxin, a highly toxic chemical, 1,000 times above normal in his blood and tissue.

“There is no doubt about the fact that Mr Yushchenko’s disease has been caused by a case of poisoning by dioxin,” Zimpfer said. “It would be quite easy to administer this amount in a soup. We suspect involvement of an external party, but we cannot answer as to who cooked what or who was with him while he ate.”

Yushchenko’s wife, Katerina, said she knew from the beginning he had been poisoned. “We had received threats before it happened and we continued to receive threats because I think there are many people who consider my husband and the changes he would bring to Ukraine a threat to them personally,” she said.

Yushchenko, wearing a scarf in his orange campaign colour, declared: “Everything is going well. I plan to live a long time and I plan to live happily. I am getting better health every day.”

The clinic’s findings look certain to be dimissed, however, by supporters of Yanukovych, who insisted the campaign was fair. Ukrainian authorities have given a number of improbable explanations for Yuschenko’s condition, among them bad sushi.

The bitter election campaign has threatened to divide Ukraine between supporters of Yushchenko — concentrated in Kiev and the Catholic west of the country — and backers of Yanukovych in the Russian-speaking east.

Yushchenko’s supporters, who see Ukraine’s future in Nato and the European Union rather than in the Russian sphere of influence, suspect he was targeted at a dinner late on September 5 at the summer house of Volodymyr Satsyuk, the first deputy chairman of the SBU, the Ukrainian intelligence service.

The organisation was formed from the Ukrainian branch of the former KGB and many members still pine for the days of the Soviet Union.

The American-born Katerina Yushchenko said she had tasted poison on her husband’s lips when he returned home. “I tasted some medicine on his breath, on his lips, and I asked him about it. He brushed it away, saying there is nothing.”

The next morning Yushchenko complained of headaches, a recognised symptom of dioxin poisoning. They got worse and he was rushed to hospital for treatment.

Serhiy Hrabovsky, a Ukrainian journalist, said: “Many politically active people have died suddenly in Ukraine since independence in 1991 of heart attacks brought on by suspected poisoning and in staged car accidents.”

Earlier this year Anna Politkovskaya, a Russian journalist well known for her criticism of the Kremlin for human rights abuses in Chechnya, claimed a severe illness she suffered was caused by poison administered by Russian agents.

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