In a stunning twist to one of post-Soviet Ukraine's most notorious scandals, President Yushchenko did not provide evidence for his claim that David Zhvania participated in administering him a massive dose of dioxin that sickened him and left his face badly scarred.

But President Yushchenko's statement raised the stakes in the four-year-old probe of the poisoning, which so far has failed to yield tangible results.

President Yushchenko, while campaigning for the presidency, fell gravely ill after attending a dinner in September 2004 with Mr Zhvania, which was hosted by two top security officials. Doctors in a respected Austrian clinic diagnosed his illness as severe dioxin poisoning.

This summer, Mr Zhvania angered President Yushchenko by claiming that the president suffered only from food poisoning and that his staff invented a politically-motivated attack to boost his popularity during the closely fought presidential campaign.

Mr Zhvania is a member of parliament representing the pro-presidential Our Ukraine-Self Defence bloc.

Asked at a news conference whether he thought Mr Zhvania took part in the poisoning, President Yushchenko answered: "I think yes, to put it mildly."

Mr Zhvania stood firm, reiterating that President Yushchenko's poisoning has yet to be proved.

He dismissed Yushchenko's statement as "absolutely ill-considered and irresponsible" and said it shows his disregard for the rule of law.

"Such actions of V Yushchenko don't disgrace him personally as much as they humiliate Ukraine in front of the international community," Mr Zhvania said in a statement.