KIEV, Ukraine – Ukrainian prosecutors reopened their investigation into allegations Viktor Yushchenko was poisoned after doctors treating the opposition leader confirmed he had been slipped the toxic chemical dioxin, as Mr. Yushchenko returned home yesterday to campaign for this month’s presidential runoff vote.

Mr. Yushchenko said he did not want the poisoning issue to overshadow the December 26 election, but the director of Vienna’s elite Rudolfiner clinic, however, said a potential criminal case could be involved.

“We are not dealing with simple pimples, we are dealing with a poisoning and the suspicion of third-party involvement,” Dr. Michael Zimpfer said, referring to the disfigurement of Mr. Yushchenko’s face.

Clinic doctors said it took a newly developed test, conducted by a lab in Amsterdam, Netherlands, to determine beyond doubt that dioxin poisoning caused Mr. Yushchenko’s mystery illness in September, leaving him disfigured and in pain.

Whoever was responsible may have thought dioxin was untraceable, Dr. Zimpfer said.

“Until recently, there has been no testing available” for dioxin, Dr. Zimpfer said. “This may be one of the reasons that this kind of poisoning, if it was a criminal act, was chosen.”

Mr. Yushchenko said he expected an investigation to find the culprits, but he urged that it be conducted after the runoff to avoid influencing the results.

“I don’t want this factor to influence the election in some way – either as a plus or a minus,” Mr. Yushchenko said in Russian as he left the clinic and headed back to Kiev. “This question will require a great deal of time and serious investigation. Let us do it after the election – today is not the moment.”

Later, after returning to Kiev, Mr. Yushchenko said “soon we’ll know who did it.” Tests showed the toxin was taken orally after likely being slipped into something Mr. Yushchenko ate or drank.

“This is the first case internationally where the intake has been oral. Usually it’s inhaled, it’s very different,” Dr. Zimpfer said. The Amsterdam tests found Mr. Yushchenko’s blood contained more than 1,000 times the normal amount of dioxin.

Ukraine’s prosecutor general’s office said it reopened the criminal investigation it closed in November for lack of evidence. Mr. Yushchenko fell ill September 5 and was treated at the Vienna clinic twice before.

Lawmakers from Mr. Yushchenko’s party said the clinic findings confirmed that opponents wanted to assassinate or disable the Western-leaning politician rather than take the risk he would defeat Prime Minister Yanukovich in the presidential election.

Yanukovich campaigners rejected suggestions the Kremlin-backed prime minister was involved in a poisoning attempt. There is “no logic in such an accusation,” said Mr. Yanukovich’s campaign manager, Taras Chornovyl.

Ukraine’s Supreme Court ordered the runoff after ruling that fraud in the November 21 runoff gave the election to Mr. Yanukovich, the hand-picked candidate of outgoing President Kuchma.

Parliament also has adopted electoral changes aimed at preventing fraud in return for handing over some presidential powers to lawmakers, easing political tensions in the former Soviet republic. Dioxin is a byproduct of industrial processes such as waste incineration and chemical and pesticide manufacturing, and the chemical is known to cause cancer.

The massive quantities found in Mr. Yushchenko’s system caused chloracne, a type of adult acne caused by exposure to toxic chemicals. The condition is treatable but can take up to three years to heal. Dr. Zimpfer said Mr. Yushchenko’s treatment will be “very difficult and long.”

While high concentrations of dioxin remain in Mr. Yushchenko’s blood, doctors said his organs have not been damaged and he is fit for the campaign trail.

“He has almost made a complete recovery,” Dr. Zimpfer said. “His liver is fine, his pancreas is fine, but he still has residual pain” and is taking painkillers.

Dr. Nikolai Korpan, the physician treating Mr. Yushchenko, said it was too early to tell what other health problems he might develop.