The allegation by sources close to Yushchenko, 50, emerged as doctors in Vienna confirmed that the politician had been poisoned with dioxin as he campaigned for the Ukrainian presidency.

Dr Michael Zimpfer, director of a private clinic that treated Yushchenko when he fell ill six weeks before the first round of voting, said that the poison, which is soluble, may have been in the politician’s soup.

“It would be quite easy to administer this amount in soup,” said the doctor, whose tests showed that Yushchenko had 1,000 times the normal level of dioxin in his body.

He had been in a critical condition, the doctor added: “If this dose had been higher it may have caused death.”

Yushchenko’s aides suspect an attempt to kill him or knock him out of the election race was made at a dinner on September 5 at the summer house of a senior official in the SBU, the Ukrainian intelligence service.

The sources said they believed that a member of the SBU had carried out the poisoning at the Kremlin’s instigation. “We are convinced Russia was behind it,” said one.

Yushchenko was forced to halt his campaign for treatment and returned from hospital badly disfigured — apparently by chloracne, a condition associated with dioxin poisoning.

The election run-off last month was won by Viktor Yanukovych, the Russian-backed prime minister, but after mass protests and evidence of ballot-rigging the result was cancelled by the supreme court. The poll will be rerun on Boxing Day.

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