“This is the work of the government,” he said. “If the prosecutor general keeps to the law and acts accordingly, then soon Ukraine and the whole world will find out who did this.”

Just 24 hours beforehand, he’d refused to name any specific officials and said the inquiry by the prosecutor general’s office should determine the culprit.

“This is a very delicate question and one should not be accusing anyone at this point — this should be determined by the investigation,” he on Sunday.

His change in opinion came after his return to Kiev from an Austrian clinic where doctors confirmed he’d been poisoned with a massive amount of toxin that can cause cancer and death.

Doctors believe Mr Yushchenko had somehow taken a dioxin dose that exceeded 1,000 times the safe limit — possibly by eating spiked soup — causing his face to be disfigured weeks before the first round of a presidential vote.

Before the confirmation, presidential administration authorities suggested Mr Yushchenko may have fallen ill from eating spoiled food or perhaps drinking too much bad liquor.

The poisoning of the pro-western politician continues to overshadow the country’s presidential election campaign.

Mr Yushchenko has previously claimed the hardline regime of outgoing President Leonid Kuchma poisoned him to take him out of the contest against Prime Minister Viktor Yanukovich.

Mr Yanukovich says he couldn’t have known anything about the alleged government plot because as prime minister, he did not oversee the security structures that could have possibly orchestrated the attack.

“In my job as prime minister, I did not have anything to do with the work of law enforcement agencies,” he said.

“I am insisting that law enforcement agencies get to the bottom of this,” he said when asked to respond to Yushchenko’s charges. “I sympathize as a person with Mr Yushchenko that he is sick. I hope that he gets better soon.”

It’s a view replicated in Washington, with the US calling for a full investigation.

“It’s terrible news to hear, and it’s certainly disturbing reports. And I know the Ukraine government is investigating this matter fully, as they should,” said White House spokesman Scott McClellan.

The Bush administration has denied claims it directly supported the Western-leaning Yushchenko.

“We have provided assistance to organizations supporting such a process in Ukraine, and that includes NGOs (non government organizations), as well as other international organisations, but that’s to assist in voter education and political party training, and it is also assistance which is made available to all political parties in Ukraine”

Asked whether he was sure that no US assistance ended up directly helping Yushchenko, McClellan replied: “Yes.”

“Our assistance is to support free and fair elections that reflect the will of the people, and that’s what we do all over the world,” he said.

But the US State Department declined to comment on Mr Yuschenko’s accusation that Kiev authorities were behind the poisoning.

“The doctors have said it was an intentional poisoning, so that needs to be looked at very carefully and there are a variety of proposals for investigation,” said spokesman Richard Boucher.

Ukraine will return to the polls on 26th December after the Supreme Court stripped the prime minister of his victory in the November 21 runoff because of massive ballot fraud.