A criminal investigation into the poisoning of the Ukrainian opposition leader Viktor Yushchenko began yesterday after tests showed that his blood contained 1,000 times the normal level of dioxin.
The poisoning caused the severe chloracne which led to the disfigurement of his face.
The results of the investigation are likely to prove politically explosive amid feverish speculation that Mr Yushchenko was the victim of a Cold War-style poisoning by members of the country's intelligence services.
The inquiry, announced by the Ukrainian authorities last night, will be led by a prosecutor general who recently took on his post as part of a deal between the opposition and the authorities aimed at ending the country's political stand-off.
Several officials have recently defected from the government to Mr Yushchenko's camp, increasing the likelihood that the truth behind the apparent attempt to kill him will be made public.
Last night Mr Yushchenko, who returned to Kiev from a private clinic in Vienna with his wife, Kateryna, and baby, sought to draw a line under the case until after a new round of presidential elections on Dec 26.
"I don't want this factor to influence the election in some way, either as a plus or a minus" he said. "This question will require a great deal of time and serious investigation. Let us do it after the election. Today is not the moment."
He went on to praise the tens of thousands of Ukrainians who demonstrated for weeks in sub-zero temperatures until the authorities agreed to a new vote. "We haven't seen anything like that for the past 100 years. I think it would be appropriate to compare this to the fall of the Soviet Union or the fall of the Berlin Wall."
Viennese doctors said at the weekend that tests showed Mr Yuschenko had ingested a near-fatal dose of dioxin, probably in his food or drink.
Confirmation that he was poisoned is likely to improve his chances in the run-off. Doctors have given him the go-ahead to return to the campaign trail but said it may be years before his face returns to normal.
The disclosure that Mr Yushchenko had ingested dioxin, an extremely rare poison, has increased suspicion that the regime of the outgoing president, Leonid Kuchma, which had the most to gain from his demise, may have been involved.
Mr Yushchenko fell mysteriously ill in September after a late dinner at the country house of Volodymyr Satysuk, the first deputy chairman of the SBU, Ukraine's intelligence service and the successor to the KGB.
Other senior security officials were present including Ihor Smeshko, the head of the SBU. Mr Yushchenko began suffering agonising abdominal pains the day after the dinner. He partially recovered but his face has been left bloated, covered in lesions and turned bluish-grey.
A Ukrainian parliamentary investigation concluded that there was no evidence he had been poisoned and suggested that his liver became infected after the party ate sushi that had not been refrigerated properly. Government officials privately dismissed his claims that he had survived an assassination attempt, saying he had probably drunk too much brandy or contracted herpes.
However, suspicions that members of the intelligence services could have been involved grew yesterday after doctors disclosed that the poisoning was only confirmed because of a newly pioneered test and would otherwise have been untraceable. Dr Michael Zimpfer, director of the clinic in Vienna that treated Mr Yushchenko, said it was only after blood and tissue samples were sent to Amsterdam that doctors were able to confirm their suspicions.
"Until recently there has been no testing available. This may be one of the reasons that this kind of poisoning, if it was a criminal act, was chosen." Some opposition politicians have voiced suspicions that the Russian security services may have been involved in an attempt to incapacitate Mr Yushchenko, who has called for closer ties between Ukraine and the European Union and Nato.
The Kremlin campaigned hard for Mr Yanukovich ahead of the elections and President Vladimir Putin twice visited the former Soviet republic. At least one Russian analyst has disputed the diagnosis offered by the Viennese doctors.
Yuri Ostapenko, the head of the Russian health ministry's poison centre, said: "Dioxin is not a poison with 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."
Yushchenko: Russia blocking poisoning probe
By Bonnie Malkin and agencies, September 12, 2007
Mr Yushchenko before and after the poisoning
Ukrainian President Viktor Yushchenko has said officials in Russia were hindering an investigation to determine who was behind his poisoning during the 2004 presidential election campaign.
The president told The Times Russian laboratories were refusing to provide samples of the dioxin poison, even though he had discussed the matter with Kremlin leader Vladimir Putin. He also said Russia was refusing to extradite three suspects.
"Three laboratories in the world were producing dioxin of this formula. It is very easy to determine the origin of the substance; there is nothing magical about it," he told the Times.
"Two laboratories provided samples but not the Russian side. This, of course, limits the possibilities of the investigation process."
Mr Yushchenko, a pro-European politician who wanted to bring his country out of Russia's shadow, fell seriously ill on September 6, 2004 as he was competing in presidential elections against a pro-Moscow candidate, Viktor Yanukovich, now prime minister.
After months of tests in an Austrian clinic, it was determined that he had ingested a massive amount of the poison dioxin.
Although he survived, his face was left bloated and pockmarked, and he has had to undergo regular treatment to rid his body of the toxin.
In an interview with Le Figaro he said he believed the dioxin used to disfigure him was made in a Russian lab.
Mr Yushchenko did not directly accuse the Russian government of being behind his poisoning, but he did say he had "practically put all the pieces together" and the attempt against him "was not a private action".
"The investigators know when, what meal, where, who. There is information on three key people who are in Russia," he said, adding that he had spoken about the matter to Russian President Vladimir Putin last December.
"Since then, unfortunately, there has been no response. I am convinced that after these people are questioned the facts will be proved."
Mr Yushchenko told Le Figaro that his pro-European instincts were unchanged, and that he still intended to have his country one day join NATO.