Ukraine's security service (SBU) has insisted it had nothing to do with the poisoning of presidential candidate Viktor Yushchenko.
Yushchenko's disfigurement could take two years to heal
Scientists in Vienna say tests show the illness and facial disfigurement suffered by the opposition leader was caused by pure TCDD - a harmful dioxin.
Mr Yushchenko, standing in the election re-run on Sunday, fell ill after a meeting with SBU chiefs in September.
But the SBU said people should not jump to conclusions.
Ukraine's parliament has reopened its inquiry into the poisoning claims. The first probe said the presidential contender had had a viral infection and other diseases.
In a statement on its website, the SBU said: "Due to numerous media reports on the place and time of possible poisoning of presidential candidate and people's
deputy Viktor Yushchenko, the SBU press centre considers it necessary to state that the SBU had nothing to do with the deterioration of Yushchenko's health.
"The SBU has not received any official documents providing legal evidence of the place, time or the poisoning itself."
The security service said it would participate in the investigation into the poisoning.
Earlier, Mr Yushchenko told the Associated Press that he was probably poisoned at a 5 September dinner with the head of the SBU, Ihor Smeshko, and his then deputy, Volodymyr Satsyuk.
Mr Satsyuk was later sacked by President Leonid Kuchma - prompting the parliament to launch another investigation over possible links with the poisoning affair.
Mr Satsyuk has insisted he had nothing to do with Mr Yushchenko's illness.
"Not only do I deny all these dishonest charges directed at me, but I intend to make sure that those who are spreading them are held accountable before justice," he told Kiev newspaper Stolichnye Novosti.
Fears of trouble
Meanwhile, the candidates are continuing intensive campaigning ahead of Sunday's vote.
The second round of the presidential race is being re-run after the original election last month - which gave victory to Prime Minister Viktor Yanukovych - was declared fraudulent, and the supreme court ordered a repeat ballot following huge opposition protest rallies.
Both candidates have said they fear trouble during the vote.
At a mass rally in the Ukrainian capital, Kiev, on Wednesday, Mr Yushchenko warned that adversaries of change were prepared to disrupt the ballot.
"Everyone must come out so that the result is totally convincing, so that there is no temptation to cheat or disrupt the ballot," he said.
"The vote on 26 December will be no easy stroll. There remain forces today prepared to disrupt it."
In eastern Ukraine, Mr Yanukovych warned supporters that emotions were running high. He said he feared possible provocation, even bloodshed, on election day.