Yushchenko, who faces Prime Minister Viktor Yanukovich later this month in a re-run of November's rigged election in the ex-Soviet state, has long alleged he was poisoned as part of a plot to kill him.
His illness kept him out of the early stages of the campaign and left his face bloated and pocked.
"There is no doubt," Dr Michael Zimpfer, president of the Rudolfinerhaus clinic where Yushchenko is undergoing treatment, told a news conference.
"There were high concentrations of dioxin, most likely orally administered."
It was impossible to determine, Dr Zimpfer added, how the poisoning had taken place. "We weren't there and we will leave that to the legal authorities to decide".
Zimpfer said dioxin is soluble and would therefore be easy to administer in something like a cream soup.
"We suspect a cause triggered by a third party," he said.
Yushchenko was well known for
his good looks until his illness
Yushchenko arrived at the clinic on Friday evening and was to undergo further testing over the weekend.
In anticipation of the announcement, Yushchenko's US-born wife Kateryna told reporters she was certain the doctors would confirm her long-held belief that her husband was poisoned.
"I know in my heart that he was poisoned and it will be medically established today," she told reporters outside the clinic.
Yushchenko's wife said this week in a US television interview that the day before he fell ill, she tasted poison on his lips when she kissed him, but he brushed off her concern.
Yushchenko says that after attempting to poison him, his enemies used mass fraud to try to steal the 21 November run-off vote, in which Yanukovich, backed by outgoing President Leonid Kuchma and neighbour Russia, was declared the winner.
The Ukraine supreme court struck down the result on grounds of irregularities and ordered a new vote on 26 December.
A third round of voting has been
ordered for 26 December
Yushchenko told reporters on Friday his health was improving and he was optimistic he would achieve victory in the re-run against Yanukovich.
"Everything is going well," he told reporters outside the clinic, his wife acting as interpreter. "I plan to live for a very long time. I plan to be very happy. I am gaining better health every day."
Earlier in the week, he said it was his "growing conviction that what happened to me was an act of political reprisal against a politician in opposition. The aim, naturally, was to kill me."
Medical experts outside Austria and Ukraine have said Yushchenko's disfiguring condition appeared to be "chloracne", commonly associated with dioxin poisoning.
Chloracne is a skin condition similar to acne that is caused by exposure to toxic chemicals such as dioxins. It usually results from occupational exposure to compounds in fungicides, insecticides and herbicides.