Sources close to Roman Abramovich say that he had symptoms of suspected poisoning at a meeting in Kyiv.
The owner of the football team had skin peeling and sore eyes.
Two Ukrainian peace negotiators were said to have been affected.
According to the Wall Street Journal, hardliners in Russia wanted to sabotage the talks.
According to the paper, the condition of Mr Abramovich and the Ukrainian negotiators has improved since the incident on 3 March.
A source close to Mr Abramovich told the BBC that he had been poisoned. They said that he had recovered and was continuing with negotiations to end the war.
At the beginning of the month, Mr. Abramovich traveled between Moscow and Kyiv.
The Ukrainian leader was not affected by the meeting between the Russian billionaire and President Zelensky.
The investigative journalism group Bellingcat said that Mr. Abramovich and the negotiators had symptoms consistent with poisoning with chemical weapons.
Eye and skin inflammation and piercing pain in the eyes were some of the symptoms.
The businessman was pictured at the Tel Aviv airport on March 14.
A spokesman for Mr Abramovich said he was playing a limited role in trying to find a peaceful resolution between the two countries.
The UK government has imposed sanctions on Mr. Abramovich over his alleged links to Russia's President Putin.
More details will be published shortly. The page needs to be refreshed for the full version.
You can get breaking news on your phone or computer. You can get the latest news on the micro-blogging site.