Doctors at Vienna’s exclusive Rudolfinerhaus clinic are within days of identifying the substance that left Mr Yushchenko’s face disfigured with cysts and lesions, Nikolai Korpan told The Times in a telephone interview.

Specialists in Britain, the United States and France had helped to establish that it was a biological agent, a chemical agent or, most likely, a rare poison that struck him down in the run-up to the presidential election, he said. Doctors needed to examine Mr Yushchenko again at the clinic in Vienna to confirm their diagnosis but were in no doubt that the substance was administered deliberately, he said.

“This is no longer a question for discussion,” Dr Korpan said. “We are now sure that we can confirm which substance caused this illness. He received this substance from other people who had a specific aim.”

Asked if the aim had been to kill him, Dr Korpan said: “Yes, of course.”

Proof that Mr Yushchenko was deliberately poisoned would be a devastating blow for his rival, the Prime Minister, Viktor Yanukovych, as the two candidates prepare for a repeat of a presidential run-off on December 26.

It would raise questions about whether the poisoning was ordered by Mr Yanukovych, his allies, or even the Kremlin, which fears that Mr Yushchenko will take Ukraine out of its sphere of influence by joining Nato and the EU.

Mr Yushchenko had said recently that he would soon reveal proof that his opponents had tried to assassinate him, but a spokeswoman said he had no plans to travel to Vienna.

Mr Yushchenko fell ill on September 6 and was rushed to Rudolfinerhaus four days later with severe abdominal pain and lesions on his face and trunk. His liver, pancreas and intestines were swollen and his digestive tract covered in ulcers, but doctors could not explain the symptoms. Against their advice he went back on the campaign trail after a week, but returned to the clinic two weeks later with back pain.

Again he returned to campaigning, with his face half paralysed and a catheter inserted in his back so that doctors — still baffled — could inject painkillers into his spinal column.

Rudolfinerhaus doctors had previously said that they did not have medical evidence to back up or rule out deliberate poisoning.

Mr Yanukovych’s supporters ridiculed the opposition, saying the illness was probably caused by bad sushi, too much cognac or a severe case of herpes. A parliamentary investigation found no evidence of poisoning.

But Dr Korpan said that toxicologists and other experts at laboratories in Britain, the US and France had since examined Mr Yushchenko’s blood samples and medical records. “We will reveal the results in the near future and confirm the cause of this mysterious illness,” he said. “We need to check him again here in Vienna. If we received him today, we could finish the whole investigation in two or three days.”

He declined to say exactly what the substance might have been, or where it might have come from. “Maybe it was administered through injection, maybe in water, maybe through eating, but the way to give it to him is very simple. This substance can be given very precisely — to only one person,” he said.

John Henry, a prominent British toxicologist, has suggested that Mr Yushchenko’s symptoms were consistent with dioxin poisoning, which causes a severe form of acne called chloracne. Doctors at Rudolfinerhaus did not initially test Mr Yushchenko for dioxin, in part because his skin changes were not as severe as they are now. He also refused a biopsy of his face because he did not want to campaign with stitches. Other doctors have suggested that Mr Yushchenko may have been struck down by a rare illness.

Dr Korpan said that Mr Yushchenko should be able to make a full recovery but needed to get back to the clinic. “We are waiting to check him as soon as possible and then we can say what he needs.”

He said Mr Yushchenko was no longer having injections, but was taking a combination of vitamins and medicine to boost his immune system.

The offices of Leonid Kuchma, the outgoing President, and Mr Yanukovych did not comment on Dr Korpan’s disclosure.

But Viktor Pinchuk, Mr Kuchma’s billionaire son-in-law, said in a recent interview he did not believe that there was evidence that Mr Yushchenko was poisoned.

“I believe he is sure it was poisoning. He’s not a liar. But some people from his camp created this provocation against him, his image, the government and the country,” he said.

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