Ukrainian President Viktor Yushchenko says Russian officials are hindering an investigation to determine who poisoned him with dioxin.
Mr Yushchenko's appearance changed almost overnight
The poisoning made him seriously ill, scarring his face in the run-up to the 2004 presidential election.
Mr Yushchenko said a Russian laboratory was refusing to provide samples of the dioxin it produces.
Asked whether he believed there was a state role in the murder attempt, he said: "This was not a private act".
He was speaking in an interview with journalists from the BBC, the Times and other Western media.
Mr Yushchenko, a pro-Western liberal, said he had even raised the poisoning issue with Russian President Vladimir Putin.
He complained about Russia's failure to return to Ukraine three people wanted for questioning. No-one has ever been charged with the poisoning.
Moscow has not yet commented on Mr Yushchenko's allegations, but it has previously denied any involvement.
After the poisoning, Mr Yushchenko was rushed to an Austrian clinic, which found he had ingested a large quantity of TCDD, the most harmful known dioxin.
At the time he was in an election race against Viktor Yanukovych, who was backed by Mr Putin.
Asked directly whether Russia was responsible, Mr Yushchenko said: "If I respond to that question, then the investigation will have nothing to do. We need to question the people who had direct involvement in the case."
He said three laboratories in the world were producing the type of dioxin used in the attack.
"Two laboratories provided samples, but not the Russian side. This of course limits the possibilities of the investigation process," he said.
He said he first fell ill after having dinner with Ukrainian security service chiefs. They have denied any involvement in the poisoning.
Looking on the verge of tears, Mr Yushchenko said no-one could imagine what the past three years of "unbearable suffering" had been like, the BBC's Helen Fawkes reports from Kiev.
He said it was only in the past few months that he had been able to stop taking painkillers every day.